The price also depends on what era machine you are looking for. If you are looking for a shiny modern one, you will probably be looking at 200-500 sometimes more or less depending on theme, condition, etc. I never owned one, so I can't help there. A Star Wars one will sell for much more than something based on a Japanese TV show that isn't known in the west.
The vintage ones you can find to be relatively cheap and plentiful if you look around. They are mostly mechanical. I bought one for $20 a few months back. It works pretty well. I have inherited others that were too damaged to even bother with unless I really get into them as a hobby.
Collectors look at things like the design rarity and placement/style of the flowers, spinners, etc. I am too casual on the topic to tell you about that stuff. They will be able to spout things like they have a ultra rare '73 Sanyo with rainbow clovers or junk like that. I know nothing about that collector stuff.
So here is about all I know:
Mechanically broken Machines:

Try to avoid unless you need parts or want to do a repair for fun/to learn on. There are too many working machines available.
Electrically broken Machine: OK for the vintage ones. They are 98% mechanical, and the electrical is pretty much lights. Easy to fix, and the machine doesn't even need it to work.
Missing glass: OK. You can get a new piece cut to size easily. I think most people use plexi.
Pachinko Balls: Getting a machine with good unpitted, non-rusting balls is a big plus. They otherwise need to be ordered and with shipping, it does cost money. Don't play old balls. It will be bad for your machine. If you can get balls in a real pachinko ball tray, that is a double plus.
If you are getting an old machine, you will usually to clean it head to toe. That can also mean taking it apart. They really can shine up nicely with the right attention. Give yourself a couple weeks for cleaning. The nice part about a cleaning is you learn exactly how they work.
Other than checking the mechanics, when looking a machine over, just make sure the spinners are not trashed, the flowers look working, and the chrome metal parts are in decent shape. Certain things like if the jackpot is working I think are difficult to check before you buy.