And any trade of Bass assumes we'd be getting a more capable big in return.First of all, this is questionable. Why would anyone trade you a better version of Brandon Bass for Brandon Bass. Trades don't happen this way.
Second, the main problem here is we need both Bass (or someone like him) and another capable big man, which to me is a clear indication Bass is not going anywhere unless it's a multiplayer trade with two big men coming back our way.
And the answer to your question would be a player on the last year of his deal who isn't a max type player. The team he is on has no real value in keeping him but a player signed for three years might have value.
Lets use Dallas and Kaman as an example. He has one year on his contract left and Dallas probably isn't sniffing the playoffs and certainly isn't a contender in the West. What real value does Kaman have to them? Trading him for a lessor player might even help them in the lottery if you think about it.
Despite what many people around here think of Bass he is an athletic big who usually has an outstanding mid range game and is money on the free throw line. If you are building a team of fast penetrating guards who likes getting up and down the floor and want a spot up big who can sink the mid range shot who isn't a liability in the 4th quarter at the line, Bass can have a lot of value.
Now is Bass as good a player as Kaman? I'd say no way. But does that make this trade unreasonable? I'd also say no. The value of a player isn't always just which player looks better to you. There are other factors involved.