Unfortunately, these just don't come up enough for there to be any good data. The best you're going to be able to do is to look at the other for-sale posts on wallgames.com, perhaps contact the posters and see if they sold, and take a guess.
It looks like working units of various types are all over the place. Unfortunately, there's just not much data to go on. Though, looking at it, we can guess that most of those aren't selling (for instance, the Sonny & Pops from 2009 gives a link to pictures. If you follow the link, there's some indication that the owner takes the price off sold stuff, and Sonny & Pops is still up there @$500. ).
What price can you live with? Is the seller giving you ANY hints what his range might be?
If I came across a working one with a nice glass, I'd be inclined to offer $250, but I'd probably not be willing to go higher. That's just me, though.
Good luck!