I am not rebuilding a XS650, but I am restoring a 1981 suzuki gs650,(a similar bike), but you can expect to have to clean the carbs,(2 cans of carb cleaner and a screw driver) this will take you an hour or two, you will need new tires(200$), brakes(50$), depending on how it was stored, you might have oxidation build up on the side of the engine. Best way to take care of that is to start with 400 grit sand paper and work up to 2000 grit. Takes some elbow grease. Depending on how safe and what type of shape the brake line s are in, you might want to replace those(130$). A new battery is a must(50-100$) if there is surface rust on the chrome, a 0000 gauge steel wool take is right off, it is amazing how well it works. Look in the gas tank to see if there is any rust, if there is clean it out and maybe even coat it. clean the master cylinders on both the front and the back, if the rubber seals are bad, replace them, safety first. Fork seals will definitely need to be replaced, I just did mine yesterday, and it took about three hours, YouTube or a clymers manual will help you immensely. Seals will cost about 10$ and fork oil will cost about 6$. After all of that you should be ready to be on the road for 500 bucks, its well worth it and you learn a lot. The biggest rule to follow is to always get everything mechanically working before you work on the ascetics. Good luck to you.