You should be looking for $7K to $9K off a Z06 at this point.
Guy on another forum won an auction for a new 08 Z06 for $56K no joke. That's like $18K off many people didn't believe but I don't doubt it's possible. He had a sales recept.
I am/was in the same area of doubt if I wanted the M3 (DCT) or some other car such as: 911, Cayman S, Z06 or GTR.
I went with the M3 as it's the one car with a REAL back seat. I have wife and two kids so I wanted a 2+2 more than anything else. If that was not an area of need, than the Z06 surely shoots up the list.
Here is what I think, say there is a list of cars for ~$70K on one side there is an ultra luxury like a LS460 and the other is no luxury like the Lotus Exige (or such). I have to figure do I want more luxury or more sport? The M3 has a nice blend of both (more leaning to luxury goodies and useful rear seats). The M3 is not as hard edge as many cars but for me mostly used as a DD with as much fun back road blasts as possible. I am not looking for a track racer.
I believe the Z06 would be OK as a DD but the M3 better especially with the DCT.