It's hard to tell with those, I watch ebay allot on them because I have a couple pristine examples (and I only need one) So I'm probably going to sell one someday.
I see them all over the map price wise. As mentioned above condition and completness is huge. An uncracked nice plastic housing, a mint face plate and nobs, a functional player with a new belt, no burnt or spliced wires, etc. would be what I'd consider "mint". Also, console vs floor mount styles vary, typically the console styles go for more because they are most often what is seen in high dollar E-body cars. The tops are all the same and the bases interchange but finding individual bases for sale is often difficult.
At high market value based on sales I've seen this year I'd say "mint" examples including an original mic (not the later over the counter replacement which looks different) and a correct cable: $900-$1,200.00 is about right, without the mic & cable: $750.00 to $900.00.
If the condition is poor they sell forMUCH less on average, restoring them is expensive and sometimes impossible due to the delicate/intricate nature of some componants
