The values of sports franchises have gone up exponentially over the past decade, but not all teams -- and leagues -- are created equal.

See the National Hockey League's Pittsburgh Penguins, now up for sale. The team's owners, billionaire supermarket magnate Ron Burkle and former NHL superstar Mario Lemieux, are looking for north of $750 million from a would-be buyer. Even though the team is one of the NHL's marquee franchises, it's a steep price for a league that generates far less revenue than its competitors (MLB, NBA, NFL) and for a city that doesn't offer glitz or purchase power.

As a result, the sale of the team has been slow-going. So, why don't the owners lower their asking price? Sources say that Burkle and Lemieux have little wiggle room and that it's the league itself holding them back.

With the NHL looking to charge a reported $500 million fee for launching an expansion teams (in either Las Vegas or Quebec City, or both), it would look bad if Burkle and Lemieux took anything less than $750 million, sources said. They suggested that there seems to be subtle pressure being put on the owners by the league to attain that asking price and not muddy the sale of the expansion franchise.

"How are they going to ask $500 million for a team whose fate is still up in the air if they can't attain $750 million for one of the cornerstone franchises in the league?" said one lawyer who has worked the NHL and other professional sports teams on franchise sales. (This lawyer was one among several sharing similar thoughts who did not want to be named due to the sensitivity of the issue.)

The league denies the claim. "Franchise transactions are negotiated privately between buyers and sellers," said NHL executive vice president of communication Gary Meagher said in an email. "The League is typically not involved in those negotiations unless asked to by the parties."

Indeed, the Penguins is a marquee franchise, having won three Stanley Cup championships over its 48-year history in Pittsburgh. The club plays in a newly renovated 20,000-seat arena and ranked 10th in average attendance in 2015-16 season according to ESPN.com. Further, the league recently signed a 12-year, $5.2 billion Canadian television deal with Rogers Sportsnet, and the values of many of the teams have more than doubled in the past five years.

But the team is still barely profitable, said an investment banking source who has worked with the Penguins in the past. And other NHL teams that are currently up for sale or were recently sold are seeking far lower valuations. The investment banker suggested that the NHL and the team's owners were forced to include a package of undeveloped real estate adjacent to the stadium to entice buyers and bolster the value of the package.

"I had a buyer group that looked at the team, but at $750 million there isn't a lot of room for growth," said the lawyer.

The Penguins are a great example of how expectations in the NHL have gotten out of control. Burkle and Lemieux announced that the team was up for sale in June. At the time, reports cited Burkle's potential view of the Los Angeles Clippers' $2 billion sale to Steve Balmer as evidence of just how much sports team values had increased.

The National Hockey League has grown to $2.9 billion per year business, but is still small in comparison to North American counterparts. The NBA, for instance, is set to bring in a total of $5.1 billion from the 2015-16 season while the National Football League projects $12 billion in revenues this year.

Still, sources say the real hiccup in the Pens' sale is the $500 million asking price for a potential expansion team in Las Vegas, an untested market for professional sports teams. Once the league is able to sell that expansion franchise, it will have a sort of baseline to go on for future team sales, the investment banker said. But is the NHL really justified in asking for $500 million for an expansion franchise?

"The underlying financial structure of Vegas is fragile. Almost one-half of the home mortgages are still underwater," said John Vrooman a professor of sports economics at Vanderbilt University, in an email. "The secret to success in a marginal edge sports market like Vegas is revenue certainty and the non-traditional demographic is risky business. This is particularly true in the gate-revenue-reliant NHL and NBA."

An investment banking source argued that having a single team in the Las Vegas market would be a boon for the franchise as it would have no real local competition. Another source countered that the franchise is likely to face similar headwinds to those faced by teams in warm weather cities: a lack of local interest in hockey as a whole, and very little local allegiance.

Among the top 10 NHL teams in terms of valuation, the Los Angeles Kings (team value $580 million, according to Vrooman) is the only one to play in warm weather.

The most recent example of a team sale in the warm weather city came in October 2014 when the Arizona Coyotes, which were also bankrupt in the last decade, sold a 51% stake to Andrew Barroway, the founder of Merion Investment Management LP, a Philadelphia-based hedge fund, for $152.5 million, valuing the team at about $298 million.

Though the valuation was well above the $170 million Coyotes owners George Gosbee and Anthony LeBlanc paid for the team on Aug. 6, 2013 to buy it out of bankruptcy, the financials of the club still don't appear healthy.

According to Vrooman, the team lost about $4.5 million in the 2015 season on revenue of just $99 million. He pinned a value of $225 million on the Arizona club, which ranks 29th out of 30 teams in attendance. The team lost close to $400 million between 2004 and 2008 before filing for bankruptcy in 2009. According to Vrooman, the average value of an NHL franchise is $505 million, or about 3.8 times revenues.

The Carolina Hurricanes, up for sale since at least October 2014, are still working to shop either a minority or majority stake in the team. Owner Peter Karmanos is looking for a $350 million valuation.

But why buy the Carolina Hurricanes, an established franchise in a growing state, for $350 million when you can have a team that doesn't exist yet in an unproven market for $500 million? That's a question the NHL should be asking itself.