God I would love a Hawk-for-cromartie trade, but why would SD want Hawk, I know they don't have great ILB's, but he isn't really an upgrade for them. I would trade them a 3rd rounder for sure.
Cromartie is not a proven commodity but has potential, Hawk is durable and a veteran which a new system may suit him better. Both those players have a certain risk factor on how the rest of their careers pan out.
The Chargers are not going to get a star player for Cromartie, a player like Hawk is probably the best they can do. I would rate Hawk's value at a third round pick, so they would be of equal value.
I think a thing that is missed here is that Cromartie is one of few available corners available on the Market, had a new CBA been in place a few more corners would be available and Cromartie would be low on that list getting very little attention. He is only getting the attention because of lack of supply. This has to be factored in.