Probably a corroded MPU ($80ish rebuild from Barakandl, a bit higher if yours it too roached and he has a donor board,) or a bad cap or bridge. So you're looking at less than $100 to invest, just a bit of time, most likely.
A good working one is probably worth $600-900, I'm guessing; I'd say $400-800 not working, depending on just how nice it is. $400 would be minor playfield wear and cab scuffing/scratches. $800 it should be pristine.
I am JUST guessing, as that seems to be the normal range for early Ballys based on that condition, and production numbers, plus factoring in what I traded to get mine. Take my opinion as a grain of salt.