We still need pics to have an idea of what it might cost to fix the bike up. Assuming that the frame and wheels are in reasonable shape, it will not cost $500 to fix it up. You can do all the work yourself. Typically an old bike needs an overhaul and you need to replace the consumables (typically tires, cables, chain, and probably freewheel or cassette). That's less than $100 in parts plus labor.