Ann Heisenfelt/Associated Press

The Adrian Peterson drama with the Minnesota Vikings never seems to end, and as news of tension rolls in from ESPN.com's Ben Goessling, it seems paramount for the Vikings to gauge Adrian Peterson’s trade value if only so they know what kind of leverage every party has—and if they want to pull the trigger.

As a small fortune, the Vikings do have the luck of being able to discuss these trades at the NFL combine with other teams, something that is traditionally the icebreaker for NFL deals in the following season. In fact, they considered trade offers for Peterson not too long ago at last year's combine according to Bleacher Report's Mike Freeman.

As Peterson battles the NFL in the court room, his distaste for some members of the Minnesota Vikings organization has created “unease” for him and limits his prospects for return with the team. Though Peterson does not seem to have an issue with most of the team, his discomfort may center around recently promoted Kevin Warren, who actively worked to keep him off the field as the Vikings’ primary legal counsel and now chief of operations, as reported by ESPN's Adam Schefter.

Gauging trade value in the NFL is as difficult a projection as there can be. While there are the typical variables involved in an NFL trade as you would see in another sport, the massive differences in positional value are starker in the NFL, age is more volatile, and trades are so rare that it’s difficult to find real benchmarks.

Looking at running back trades in the past three years is not very helpful, either.

In 2014, one running back was traded: Darren Sproles.

The Saints received a fifth-round pick from the Eagles for Darren Sproles.

In 2013, six running backs were traded: Trent Richardson, Delone Carter, Felix Jones, Chris Ivory, Jeff Demps and LeGarrette Blount.

The Browns received a first-round pick for Richardson from the Colts.

The Saints received a fourth-round pick for Ivory from the Jets.

The Buccaneers received a seventh-round pick and Jeff Demps for Blount from the Patriots.

The Colts received David Reed, WR, from the Ravens for Delone Carter.

The Eagles received Adrian Robinson, LB, from the Steelers for Felix Jones.

In 2012, there was one running back traded: Mike Goodson

The Panthers received Bruce Campbell, OT, from the Raiders for Mike Goodson.

In addition, there have been draft-day trades involving running backs. In 2014:

The Bills received Bryce Brown and a seventh-round pick from the Eagles for a seventh-round pick and 2015 fourth-round pick.

The Browns traded up for Terrance West. They gave up a fourth-round pick and a sixth-round pick to do it.

In 2013:

The Buccaneers swapped sixths with the Vikings to grab Mike James, and threw in a seventh to do it (moving up to 189 from 196).

The Rams moved up into the fifth to select Zac Stacy. They gave the Texans two sixths.

The Packers moved up into the fourth to select Johnathan Franklin. They gave the Broncos a fifth and a sixth.

In 2012:

The Dolphins swapped fourths with the 49ers for Lamar Miller. They traded the 49ers a sixth and a 2013 sixth to move from 103 to 97.

The Ravens swapped thirds with the Falcons for Bernard Pierce. They traded the Falcons a fifth to move from 91 to 84

And in the draft by itself, the following first- through fourth-round picks have been used on running backs:

Draft Picks on Running Backs First Trent Richardson (3) Doug Martin (31) David Wilson (32) Second Giovani Bernard (37) Le'Veon Bell (48) Isaiah Pead (50) Bishop Sankey (54) Jeremy Hill (55) Carlos Hyde (57) Montee Ball (58) Eddie Lacy (61) LaMichael James (61) Christine Michael (62) Third Charles Sims (69) Tre Mason (75) Bernard Pierce (84) Terrance West (94) Jerick McKinnon (96) Knile Davis (96) Dri Archer (97) Fourth Lamar Miller (97) Devonta Freeman (103) Robert Turbin (106) Andre Williams (113) Ka'Deem Carey (117) De'Anthony Thomas (124) Johnathan Franklin (125) James White (130) Marcus Lattimore (131) Lorenzo Taliaferro (138) Pro-Football-Reference.com

The running backs that have been picked at least imply that finding the right running back is a harder than people are willing to admit, and if a general manager were to have the choice between a 30-year-old Adrian Peterson with controversy hanging over his head and a number of those players, they would easily choose Peterson. Those second-round picks have five total years as the primary starter out of 19 possible years—and 43 yards a game. For every Eddie Lacy or Le'Veon Bell, there's another LaMichael James or Montee Ball.

Whiffs in the first round, along with some misses in the second and third and uncertainty elsewhere on the second day, make it seem as if it may actually be a better bet for a team without a running back to invest short-term in Peterson and perhaps make a few picks later on in the draft to develop behind him.

This all, of course, depends on the possible cost of acquiring Peterson. Any team that trades for the former first-round pick out of Oklahoma will necessarily value Peterson at a discount versus the Vikings—after all, only one team is built to work off of Adrian Peterson's carries, and that's the team that has him.

The question of Peterson's decline is further complicated by the fact that missing a year may be a good thing, and that there aren't any signs of "decline" either on film or after exhaustive statistical work, even after accounting for the vagaries of statistical analysis.

Ann Heisenfelt/Associated Press

It's clear that NFL teams value running backs more than most modern analysts and a number of fans, and there's a reason to think that: Finding running backs may actually be harder than fans give the NFL credit for, and though running backs will never touch quarterbacks in terms of pure value, they may have more value than any other skill position despite the prevalence of the passing game. Consider the fact that running backs receive more targets and touches than any receiver by a factor of three or more.

The Cowboys are wrestling with this question right now, potentially determining who to pay between one of the best receivers in the NFL in Dez Bryant, who had 136 targets and 88 touches, and DeMarco Murray, who had 456 total carries and targets (or 449 touches).

Certainly Bryant seems harder to replace than Murray, but the Cowboys' best offensive season in total points scored since 1983 happened to come when Murray received over 450 total looks by the Cowboys offense. The last time they were better than fifth in total points scored (what they earned in 2014) was 2007, where their running backs combined for 458 total looks.

Though there's something to be said about late-game effects by winning teams, it is important to note that teams are relying on the run more than people give them credit for, with the top two teams in points per drive ranking 10th and third in percentage of plays that were run attempts. Of the top 10 in points per drive, four offenses ranked in the top 10 in percentage of plays that were run attempts.

Regardless of the actual impact running has—again, the importance of accounting for point differential and time remaining for statistics like that does matter—it's clear that the NFL values running, even if many analysts don't.

To that end, Adrian Peterson's market is well worth monitoring. There aren't many older, premiere players to look at to create an additional guide. Darrelle Revis to the Buccaneers cost them a first- and fourth-round pick (and at the time, more likely a third-round pick) after he was recovering from an ACL surgery at the age of 28, and Brandon Marshall to the Bears at the same age cost two third-round picks fresh off of serious concerns with regards to his mental health and a number of legal troubles, including an accusation of assault two days before the trade.

Though they both play an important role in a more important facet of the game (passing), the degree to which an individual skill player is important is smaller in the passing game than in the running game. Even then, if it were the case that Revis or Marshall could help a team more than Peterson could, they can provide a decent guide to project what Adrian Peterson can do.

Whichever team trades for Peterson will believe that he at least has a number of years left as a high-level player, and they can say that with more confidence than many could have said about Revis, who was traded while he was still recovering before he showed on-field return to form. Peterson is also recovering from a legal issue far removed from the event in question, unlike Marshall.

On the other hand, this draft is purported to be one of the better ones in recent years for running backs and could further depress Peterson's value.

As a result, it may be wise to hedge bets and split the middle with regards to the value of those two trades on the draft value chart, acknowledging that future picks (like the conditional pick for Revis and one of the two third-round picks) are generally discounted by one round (and it helps that both trades occurred when we knew the specific pick in question for that year).

Tony Gutierrez/Associated Press

That may account for a little more than 500 points of value on the chart, which would be a second-round pick for most teams, and often an additional pick. For example, the Cowboys would want to trade a second- and a third-round pick to meet that value.

That does seem a little high, even after acknowledging the specific value of running in the Cowboys offense and Jerry Jones' well known fondness for Adrian Peterson.

Knowing that this is a strong draft for running backs, Adrian Peterson's value may be closer to a second-round pick and a fourth-round pick, or even a future fourth-round pick, conditional upon playing time (that may move up to a third or move down to a fifth on a sliding scale). Two third-round picks, a la Brandon Marshall, would not be out of the question, either—though likely is a little low.

It's naturally difficult to project what will happen with Adrian Peterson, but his trade value may be higher than many think. Whether or not the Vikings would rather take that and offload a portion of his contract instead of having what could still be the best running back in the league discontented (and maybe holding out) is a different question.