1. Boban's unique salary cap provision and the 2016 team cap situation have been discussed at length in this thread:
http://www.spurstalk.com/forums/showthread.php?t=256965
2. Google for "Gilbert Arenas provision" if you don't want to read through that thread.
3. The NBA salary cap will explode this summer, with 20 or more teams having max salary slots. There are only a handful of players who really deserve top dollar. After those guys sign, a bunch of average players will get big contracts.
4. Even under a much smaller cap, Asik got 4/40 and Ed Davis got 3/20, for just two examples.
5. Boban will very likely command a contract that starts at the MLE and could have huge back end payments due, and even an MLE-sized contract could be problematic for the Spurs.
Bottom line, the Spurs will be able to keep Boban if they choose to, but it will be expensive.