The biggest reasons for the New England Patriots' sustained excellence over the last 15 years are Tom Brady's quarterbacking prowess and Bill Belichick's coaching brilliance. But a close second is the fact that nearly every one of their stars plays for less than he's worth.

Brady, who recently inked a two-year contract extension, sets the tone. The new deal will pay Brady $30 million over the next two seasons, which pales in comparison to the $37 million Brock Osweiler will receive from the Houston Texans over that same time period. Despite his stature as one of the greatest quarterbacks to ever step onto the gridiron, Brady has never been the highest-paid player at his position -- not even close. Last season, Brady made $9 million, whereas Aaron Rodgers brought home $22 million. Brady was the 21st highest-paid quarterback in 2015, just behind Nick Foles and ahead of Jameis Winston.

Brady's hometown discount sends the message that Patriots players are often expected to sign and honor below market deals. One of the most underpaid players on the roster today is Julian Edelman, whose four-year, $17 million contract is considerably less than what he should be earning.

Edelman, 29, signed this deal following his breakout 2013 campaign, when he caught a career high 105 passes. He's been Brady's No. 1 target since, catching 153 balls in 23 games over the next two seasons (Edelman reeled in the game-winning grab during Super Bowl XLIX as well).

In addition to increasing his production on the field, Edelman's brand has blown up off of it. The 2009 seventh-round draft pick out of Kent State is now a football alpha dog, starring in popular video shorts such as Burgertyme and landing a front row seat at New York's Fashion Week. Yet, he hasn't publicly expressed any desire to renegotiate his contract.

If Edelman did, he would surely have a lot of leverage. When he was on the field last season, the Patriots converted more than 50 percent of their third down attempts. In the nine games without him, the Pats were successful on just 31 percent of their third-down plays.

The first week of free agency alone, Mohamed Sanu, Marvin Jones and Travis Benjamin signed contracts worth a combined $96.5 million. Given the current state of the market and volatility of slot receivers, it's more than surprising Edelman hasn't pushed for his deal to be restructured.

Rob Gronkowski did comment on his contract last week, although it may have been in jest. Following the Patriots' decision to exercise the remaining $6 million on his $10 million option bonus, Gronkowski appeared to tweet out his displeasure about his pay relative to other tight ends.

If ya think about it that Option pick up basically equals pay cut the next 4 seasons... I don't work hard for those reasons. Haha — Rob Gronkowski (@RobGronkowski) March 8, 2016

Though the high number of emojis makes it difficult to assess Gronk's seriousness, he does have a valid argument. He will make less next season, for example, than Jimmy Graham, Travis Kelce, Julius Thomas and Zach Ertz. Gronkowski's minimum salary for 2016 is $8.5 million, which is a step below the $9 million he brought in during 2015.

The contracts a couple of mediocre free agent tight ends have signed recently may irk Gronkowski, too. The Colts inked Dwayne Allen to a four-year, $29 million deal last week, even though he's coming off a 12-catch campaign. Coby Fleener, Allen's former teammate, signed a five-year, $36 million contract with the New Orleans Saints. Their annual salaries are comparable to Gronkowski's, but their production doesn't come anywhere close.

At the time, it seemed like a wise decision for Gronkowski to a sign a six-year, $54 million extension in 2012. He was only two years into his NFL career and had an extensive injury history. Gronk missed the entirety of his junior season at the University of Arizona due to back surgery, which caused him to slip to the second round of the draft. He also went under the knife for an ankle injury after Super Bowl 46 and broke his left arm twice during the 2012 season.

But circumstances change over time. That's especially the case with Malcolm Butler, who was an undrafted rookie two years ago and is now the Patriots' No. 1 cornerback. The defensive backs at the top of the market, Darrelle Revis and Janoris Jenkins, have signed deals the last two offseasons that pay them $39 million and $29 million guaranteed, respectively. Butler, meanwhile, is set to make just $600,000 in 2016.

To put it bluntly: Butler is criminally underpaid. His predecessor, Revis, would almost surely advise him to hold out over training camp this summer. But so far, that doesn't appear to be in Butler's plans.

Throughout Belichick's tenure, the Patriots have used the allure of yearly Super Bowl contention to convince players to take massive hometown discounts. But as the pay scale across the league continues to rise, the Patriots may have to start giving into the market.

Paying a little bit more now could save them a lot of money and contention in the long run.

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