There is no objective evaluation that applies across the board to everyone. Assets have different values for different teams. Celtics have very little use for young players with potential for the next few years. Sacramento is trying to establish a winning culture. They can't handle Cousins. That being said, you underrated a lot of the Celtics assets.
Kris Joseph and Fab Melo are putting up excellent stats in the D-league outperforming many players drafted higher than them. Jared Sullinger, arguably, should have been a top 5 draft pick in the past draft. These guys are on the best possible contracts in the League. Cheap for a long time. The Celtics future first round draft picks should be expected to be around late teens to early twenties. Brandon Bass and Courtney Lee are valuable trade assets for a young team. They are relatively young, productive, good character veterans with deep playoff experience on very reasonable contracts.
Leandro Barbosa, Chris Wilcox and Jason Collins would have a lot of trade value for teams who need specialist help. A contender that needs a scorer, an energy big or post defense would value these guys highly especially on a minimum contract. LA would love to have Barbosa (Duhon, Morris), or OKC wouldn't love to have Wilcox (Thabeet). Jason Collins has a lot of value for any teams anticipating facing a Bynum or Howard in the post-season. If they thought they might help them win a playoff series or two, they might even give up a good pick or two like the Pacers last year.
Avery Bradley is extremely valuable. He's extremely young, and is a top defender. He is a monster in transition, he has an excellent shooting stroke with solid percentages last year, he's a great cutter. He's barely 22 with playoff experience. He has not had one negative incident and everyone including Rondo, Doc and KG raves about his character and work ethic.
Boston has a lot of different assets of varying value. They can make many different deals with many different teams, but the key is what are they looking for and do they have enough. The idea that Boston has no valuable assets outside of the Big 3 is ridiculous.
For those who are not clear about Rondo's trade value, look at Chris Paul trade, and compare their numbers this year. Even better look at Deron Williams trade and check their numbers. Please stop with Rondo for Al Jefferson or Evans and Cousins stuff. Neither one of those guys (Paul and Williams) have half the signature playoff series, or that ring that Rondo has.