So it looks like ignoring the reduction of income to other lower tax states (i.e., if you just pretend all of the reduction comes from the home state and you assume he didn't have additional incentive pay not being reported), Darnold would have broken even if his marginal home tax rate was 3.79%. If it's lower than that, he actually paid more in extra taxes than his extra incentive pay. If it's higher than that, he still made some money.
So basically players on teams from Florida, Tennessee, Texas, Nevada, Arizona, Indiana, Pennsylvania, Washington, would have paid more in extra taxes than they earned.
Players on teams from Georgia, Illinois, Maryland, Massachusetts, Michigan, Minnesota, Missouri, New Jersey, New York, North Carolina, WIsconsin, DC would have come out slightly ahead.
Still should be a constitutional violation for earning more money in a state to reduce your total take home pay.