To understand what is going on here, you have realize that this trade irrevocably puts the Celtics on the path of rebuilding after Ainge has been sending out mixed signals for a while.
This trade is really taking back a bunch of assets as the price of taking on the contract of Gerald Wallace. Paul Pierce is really there to match salaries. Terry is being sent over because Ainge can dump his salary in the trade. KG is being added to the deal because he's not a good fit for a rebuilding team's roster, so might as well get something for him and give him respect by offering him a chance to control his destiny and waive his no-trade clause.
This is a potential one-year rental of both Pierce and Garnett. They may be Hall of Famers, but that's still not worth more than a young player, a first pick, and expiring contracts.
So, Pierce and Garnett for two young assets each (four total), Humphries, and the first year of Wallace's contract. Then Jason Terry for the second and third year of Wallace's contract and any other assets involved. Getting some of those picks to be unprotected increases the value of what is involved.
It's a shame that the Nets have traded away every second-round pick through 2017, since it seems they undervalue picks.