I've been an advocate of getting Ryan Anderson, if possible, and see $9-12m/year as a reasonable price range.
I love a good stretch four who actually makes his threes at a high rate and think that kind of player paired with KG up front would create the sort of offense that would maximize Rondo's usefulness. I love his shot selection. According to basketball-reference.com, through April 3, he took 636 shots. 185 at the rim, 363 beyond the arc, only 36 long twos from 15 feet or further. Even as a rookie in New Jersey, 88% of his shots came either at the rim or from three-point range. If you take his time as a 20-year-old rookie for the Nets as a baseline, his stats look like a normal progression of player improving as he gains experience, so I think that those who say he is a product of Orlando's system are wrong. This is a kid who would thrive anywhere.
From what I've read, he is often described as a high BBIQ player, so he seems capable of grasping the Celtics playbook, and he is described as having a good work ethic. His three-point proficiency obscures that he is capable of driving to the basket and posting up, so he has an inside as well as an outside offense.
Overall, I expect him to be capable of better numbers than he is putting up now if he finds himself somewhere other than Orlando next season. I'd take him over Javale McGee, Roy Hibbert, Eric Gordon, or Deron Williams.