I agree that Rondo's trade value is down, but it's not like it's gone. Derrick Rose went down hard, and it's not like his value went to zero. He was playing as a top 5 (or so) player in the NBA. Chicago could have traded him for something significant. Boston could get something for Rondo now, but not as much.
It's not like any offers for Rondo last year wowed Danny, so Rondo is likely sticking with the Celts for a while. That said, if we were determined to trade him, I bet we could get maybe 1/2 of what Brooklyn gave up for Deron Williams.
In that deal "The Nets will send Derrick Favors, Devin Harris, two first-round draft picks and cash considerations to Utah for the All-Star point guard." I imagine the Celts could get 1 first rounder and a good young second player back.
That said, I agree that it's not worth trading Rondo now as it's not going to put us over the top or get us full value for what he's still worth. He can return and mesh with our team next year.