1990 ZR-1 Value

[zkant]

First time posting, but I have been looking at getting a ZR-1 for awhile now and I am getting close to buying one. I'm looking to see what you guys think the one I am looking at is worth. I know it is very hard to give an estimate without seeing a car, but I will try my best to list everything I know.

It is a 1990 Red/Black ZR-1.
20,000 miles.
Second real owner, was GM exec car for 3 years at first (don't know if this does anything to value)
Paint and interior are great
Has both a clear and red top
Working radio
Everything should be original

What do you think it is worth? What should I offer at first? What other factors do I need to look for that could affect value a lot?

Any advice would be greatly appreciated! I hope to be a true member of the community very soon!