There are three iterations of Cb500/550 Fuel valves. 72-74 Cb500/550, 75 and 76 Cb550, and 77-78 Cb550. Each was an improvement on the one prior in some way. The Cb750's were improved with the same type fuel valve on the same year boundaries, AFAIK.
Expensive is a subjective term. Compared to replacing the whole bike or making the bike you have functional, they are cheap. They are priced on what the sales marketplace will bear and the demand. The 75 and 76 types are very fragile and are often destroyed by ham hands and others who have no business working on them blindfolded, making the demand go up.
How is it you ruined yours?