I have to respectfully disagree with Vettelt193. Lack of parts will actually increase the value of ZR-1's. The harder a mint car is to come by, the more valuable they become. This is especially true of a limited edition, highly collectible car like the ZR-1. Several dealers have parted out ZR-1s at a profit, and every Z off the road makes the remaining ones more valuable.
Performance commands a premium, and the ZR-1 hasn't really been KOTH since the '02 Z-06 came out. That car moved a lot of the speed freaks out of their C4s. The C6 may add a bit more supply to the ZR-1 market, but honestly, can you see prices dropping below the mid-teens that many '90 and '91 cars are fetching? I can't.
There are enough collectors and casual fans on the sidelines, that will snap these cars up at prices below $20k for good->excellent condition ZR-1s. I believe the '90 and '91s are pretty close to the bottom of their value. '92-'95 will drop a bit more, but I honestly don't see the 405HP models ever getting below $25k for a nice example (<50k, and no major issues).
There are still lots of people racing and modding these babies. Everyone of those makes the remaining cherry stock cars even more value. As time goes by and there are fewer and fewer nice examples left, the value will begin to appreciate. I would expect this to start happening in 8-10 years. In the meantime, we'll see prices drop a little, but mostly stay pretty flat.
Just my $0.02
-Burt