I had a shot at one awhile back for $100, that didn't powerup at all, had a really ugly top-fire stick on it, and needed ALOT of TLC on the art/cabinet.
I would have bought it in a heartbeat except that the guy didn't have the keys for it either, so I couldn't tell if the boards were present, wiring was shredded, monitor was intact, etc...
I didn't want to pay $100, and then have to replace everything in/on the cab afterward.
There's a good condition, working one near me that they want $700 or $800 for.
That's a little steep for me though.
I'd probably go $500 on it, if I had the money to burn.
For one needing a monitor, but in good shape otherwise, I'd probably go $300.
The monitor will cost you about $200-$300 to replace with a new one--depending on where you live.
It would cost less to replace it with a good condition used one, if you can find one locally.