You can't take Jackson's numbers over the past couple of years and use them to predict how he would be on the Spurs. He's had ty field goal percentages putting up lots of shots as the #2 or 3 option on a bad team. On the Spurs, as a role player getting open jumpers off TD post up and TP/Manu drives, he would be deadly.
Also, as far as his cost, if you trade Mason/Finley/Marcus Williams, the Spurs only take in about $550K in additional salary for this year, which would result in about and extra $1.1 million Holt would have to pay toward player salaries. It's nothing to sneeze at, but it's also not an exorbitant request.
Buford et al would just have to get a little creative in the coming years to carry Jackson's contract. It would be great for our le run this year and next, and hopefully by the time he regresses significantly his contract will be close enough to expiring that he is tradeable.