Because these cars are realtively inexpensive (compared to a Porsche Speedster in fully restored condition), say less than $28k (in most cases), their value as used cars takes a pretty good hit in the first year. The reason is simple: why pay $25k for a used replica when you can get a brand new one built to your own specs for just a little more?
However, not everyone wants to have one built. And not everyone is rational when it comes to these cars. So, depending on the condition and mileage, plus the area of the country you live in, used prices can vary significantly.
How's that for an evasive, non-answer?
Reasonably, if you pay $25k+ for a new one, take excellent care of it, drive it for a couple of years, you could probably sell it for $18k - maybe more...in the Spring...in the Sunbelt. And that opinion comes strictly from having watched the replica market for a couple of years before I actually bought (a new) one. It's not scientific fact.