I just proposed a method - take all endorsement income on a per team basis and average that out. A team's percentage over or under the league average is added to the local tax rate. That percentage is then allowable as a signing bonus to the contract value. It's ridiculously simple.
But endorsements are based on individual players, at least the ones we're talking about it. Chris Bosh isn't as marketable as LeBron James and Dwayne Wade, but if all three signed in the same team in a smaller market, the team would be allowed to offer all three of them a signing bonus based on average marketability. Back in the day, the Raptors were marketable solely because of Vince Carter. His endorsements weren't based on the fact that he played in Toronto. That's usually the way it is with big time players. More on that later.
No cost of living because a) player's are not required to live in their cities during the offseason, b) they travel 1/2 of the season and c) these guys aren't living in apartments and clipping coupons - when you reach a certain income level "cost of living" becomes "how much can I spend".
When you're talking about premiere players, sure. But it's still a factor. The wide-ranging cost of real estate throughout the country makes a big difference, whether the players live there full time or not. And even if they travel, that doesn't mean their families don't need somewhere to live. And it's pretty clear and undeniable that it costs a lot more to live in Miami or New York or LA than it does to live in Cleveland or Detroit or Indiana or Memphis. If we're worried about state tax, then why wouldn't we be worried about cost of living? It's just as varied. It could easily cost LeBron $8 million to build a home in South Beach, and that's pretty much what he's saving in tax money. It's a wash.
Besides, when have you ever heard about cost of living being a factor for why a free agent would choose one town over another? You haven't. How much did we hear about endorsement money and tax money this offseason and in the past? ALL THE FRIGGIN TIME.
We heard about endorsement money primarily because of LeBron's stated intention to be a global icon and billionaire athlete. We didn't hear too much about tax money, not that I can remember. And I really don't think it was that big of a concern when these guys made their final decisions. Amare could have gone to a team in a state with much lower taxes than New York.
And the counter argument to the endorsement issue is that when you're a big time player, you're going to get endorsements. Nike and Gatorade and Vitamin Water and all those companies don't care where you play, as long as your team is good and you get national TV slots.