I got a 1986 Motordome pinball machine for $300 last year. It needed some help (2 new transistors, 1 coil, and a few playfield touchups) but plays great now.
If you want one for under $500 that isn't an electromechanical, your best bet is to look for one of the less popular models, like Motordome. What I found is that since even fixed up, these less popular games sell for only $600 or $700, a lot of the pinball repair/sell guys won't "waste" their time on them even if it's an easy fix. They'd rather spend that same amount of time fixing a more expensive machine. I can kind of see the logic. I mean, if you only have an hour to spend working on things, do you want to spend your hour fixing the machine that will only sell for $600, or the one next to it that will sell for $1500?