It all depends on what's actually wrong with it. You initially asked for the cost of a full rebuild which can be very expensive. But like others have said, it might just need a few odds and ends to get it running nicely. Sure, it might not be fully reliable unless you really dig into it, but if you want a winter project and a sweet running saw at the end, this could be a good one.
Take a look at the exhaust side piston skirt. If it's in decent shape, it might just need some new rings. If it's scored, figure out why and keep your eyes peeled for parts.
Buying a new saw is almost always cheaper than having an old one rebuilt by a dealer, but don't let that scare you off. You'll save loads on labor if you tinker with it yourself.