The way I figure it;
Spurs should have around $10,077,921 committed to;
PG-Parker, Nico Lapr.
SG-Green, Simmons*, Murray, Forbes
SF-Leonard, Bertans
PF-Aldridge, Anderson
C-Gasol, Milutinov*
*Cap hold or QA
I think Mills is almost certainly gone and it will be almost impossible to keep both Simmons and Dedmon under that scenario.
I highly doubt that Gasol opts out of $16 million next season unless he is looking for a 2 year deal with guaranteed $. If he is, then the Spurs could offer him a 2 year deal in the area code of $24 million if he were to opt out. That could give the Spurs around $14.5 million to try and bring back two of those three.
Mills has a cap hold of $6.8 Million, leaving around $7.7 million in cap space for either Dedmon or Simmons in the second scenario. I'm not sure whom the Spurs would pick if that were to happen. Mills is a starting caliber PG, while Dedmon could certainly develop into a starting caliber center as soon as next season, while Simmons has shown that he could be the replacement to Manu as the backup SG.