I've got the OP roughly $1M over the cap at $71M for 15 players assuming Daniels, Dooling, JO, and Sasha are out and 2 first rounders and Green and Gray are in. I do not think you get RA for $4-5 million, even with a hometown discount, given he is the best UFA SG. It is a players' market this summer with not much UFA talent and tons of cap room from teams who pared salaries for the new CBA and Dwight Howard. Also think you need to do RA and KG for 2 years.
Start with $71M, subtract $8M for Green and Gray for the moment to bring it to $63M for 13 players then add $3M to Ray Allen ($7.5M annually for 2 years). Then you are at $66M. This year we're at $79M and have spent more in the past. I assume we'll spend roughly the same amount so that leaves $13M ($79M-$66M) to get one or two very good players from:
http://www.hoopsworld.com/2012-nba-free-agentsPP
KG
RA
RR
AB
BB
GS
MP (healthy)
CW (healthy)
----
9 rotation players
+1 rotation player from 2 firsts. EM, and JJJ
+2 FAs
----
12 Rotation players + 3 NDBL/inactives
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I think you can actually contend with that budget and it fits the San Antonio model.