http://www.hoopsworld.com/san-antonio-spurs-team-salary
Here is a very detail breakout of the Spurs Salary for next year and Beyond. Right now they will have 9 players under contract with a total 49mil salary. Mills probably will terminate his contract early and Green will probably be given a qualifying offer. So, at the start of FA, if Duncan isn't given an extension before then, the Spurs will still probably be at 49mil with 8 players under contract, and one restricted.
Duncan's Cap Hold is the biggest drawback right now. But let's say he does sign a 10mil a year deal, the Spurs will be at 59mil. The Cap is suppose to go up to 60-61mil.
So basically the Spurs would have to amnesty Bonner or waive Blair to afford Lorbek.
Honestly, with how good the team is doing, I say keep everything as it is. The Spurs could sign Mills with 2mil Exception, resign Green to a long term deal since the Spurs have his early Bird rights, and then resign Boris with the MLE.
So basically, especially if the Spurs win the Championship this year, they more than likely can bring the same exact team back next year if they wanted.