A good rule of thumb is to make a quick shopping list based on the worst case scenario, then subtract the value of that list + something for your time from the going rate of a functional game in similar physical condition.
- It most likely hasn't been shopped in a long time. At the very least, you'll need a set of rubbers and a flipper rebuild kit.
- If the flippers aren't flipping, although it could simply be a blown fuse, it may very well need a new flipper board.
- The display is not working and the original Sega large DMDs are very hard to come by. Because of this, you may as well upgrade to a ColorDMD.
- As mentioned above, the MPU could be in great shape, but it's possible (and more likely) that it has suffered alkaline damage. The DE/Sega power supplies are known to have issues, as well, so you can add one of those.
As you can see, it can add up pretty quickly. Keep in mind this is also just to get the game running as intended, never mind extras like LEDs. It's entirely possible that you bring it home, swap some fuses, and she'll fire right up, but keeping the worst case scenario in mind can really save you some $.