When I traded my 05 with 15,000 or so on it they gave me 23,600 for it. They also discounted my 06 C6, so I'm thinking that was pretty close to the wholesale number on it.
That number seems pretty good to me, but dealers typically mark a car up 3k over what they have in it. That means trade value plus whatever they run it throught the shop for = what they have in it. They are usually able to come down to about 1500 over what they have in it on a used car.
I'm looking at KBB trade-in and I see 23,175 for excellent and 21,850 for good. I'm betting the dealer was able to buy the car closer to good. With that being the case you might be able to get this one for 23,000 or slightly less.
Good luck and do it now.