1) Martellus Bennett is a much superior player to Julius Thomas. He does considerably more than Thomas within the confines of an offense that does considerably less.
2) The Bears aren't trading Martellus Bennett. He's on a good deal, he's productive, and they just lost a receiving threat. Why in the world would they move Bennett?
3) Even if the Bears were trading Bennett, they absolutely wouldn't do so for the peanuts you're proposing.