Rajon Rondo ($11 mil) + Kendrick Perkins ($9 mil) + Iggy ($14.7 mil) would be close to $35 million in contracts in 2012/13. Add in two first round picks in the 20s, another say $3mil. Let's say the rest of the roster is vacant, no other contracts, so a cap hold for empty roster spots of about $3.5 million. A total of $41.3 million.
The Celtics would still have a massive amount of cap space in 2012 if they built around a core of Rondo, Iggy and Perk. They could add a fourth star, hopefully an All-NBA guy, and make a Big Four. If that final piece was good enough I think that creates another contender.
If that piece wasn't good enough (if All-NBA guy not available), then I think the Celtics would need to add two quality scoring (17-20ppg) options. Of the $10mil per annum variety. Or one scoring option and one under-appreciated star (say a Paul Millsap type). In order to make a strong starting five somewhat comparable to how the Pistons built their title winning team.