I know many people refuse to consider taxes when this issue comes up, but let's make some basic assumptions (they are not true, but close) and assume that a QB makes, on average $40 million per year over 3 years. That is $120 million. Now lets assume that player pays all of his taxes in the state in which his team is based. (Doesn't happen this way, but just assume it as fact.)
Player signs in New Jersey, State Income Tax is 10.75%, so player pays $21 million in State Income Taxes over the life of the contract. (I know it is less because his per game salary is taxed where he plays and NJ has an extremely high state income tax rate, but this is the simplified version.)
Player signs in Louisiana, State Income Tax is 4.25%, so player pays $5.1 million in state income taxes over 3 years. (North Carolina is close to Louisiana and is at 4.99%)
Player signs in Florida, zero state income tax, player pays zero state income tax. (I know he will pay tax on game checks on away games, but this sort of averages out in most situations. I also think signing and roster bonuses are paid and taxed in the home state of the team.)
You have to think that at some point taxes come into play for some players and agents. The difference between New Jersey Jets and Tampa Bay Buccaneers is at least $15 million over three years, and potentially higher. That's some pretty serious money there and averages about $5 million per year. Louisiana is maybe about $3.5-4 million a year "better" than New Jersey.
What I am getting at, if Jets made offer of $40 million/year, Saints could pay $37 million and Tampa could pay $35 million/year and Carr would be about in the same financial boat. (I assume any contract would be very heavily weighed towards bonuses and that is probably taxed my the home state, so per game check varying taxes does not play a huge part.)