If you're the only one that's interested then under 500 would be typical. Otherwise, prices can climb quickly if it turns into a bidding war. What you want to do is find out all you can about the machine and what it's supposed to do so you can verify when you get there.
There are no spare parts for these so getting them fixed means taking them to someone savvy with old school electronics. Over the years I've found a couple of places to fix them but it's never a guarantee. Luckily I just found a place near me that works on this type of stuff.
Anyway, best advice is to know what it's supposed to do and play a few games to verify.
the fact that it has balls and tokens is great but that's not a big deal. Those are readily available so don't let the seller claim otherwise.
post a pic of it and chances are I've seen it and can tell you all the functions.