Looking at the photos, there is one thing that is clear to me - it is being used as shelving at the moment. Given he paid $500, I'd take $750 cash and see what he says. Have an extra $250 in your back pocket to produce if you cannot extract the game with $750. If you can't score this for $750-$100, then I think he wants to hold on to it. Give him your number, and I'll bet he calls you within 2 weeks.

One thing that is not clear is the condition of the playfield (looks to be ok), and the backglass (rear). As Marty pointed out, the boards need to be quickly looked at for acid damage.

I think if there's no issues with the backglass, playfield (hidden touch ups etc) or boards, then you can start at $750, as that is the going rate for projects. If you get a good look - and find all is clear, then $1000 would be the price.