Yeesh...hate to say it, but it's not worth a whole lot. The rollers have a ton of wear (and rusty too!), there's no ball. It'd be good as a parts unit, perhaps.
Actually, now that I think about it, as long as the optics work, you can get a rebuild kit for cheap. This is assuming, of course, that the case isn't cracked. Toss in a ball, and off you go.