I've been collecting BSA parts for my '67 A65. They are costing less then XS parts. I bought some Triumph parts as well from a guy who solely sells British parts and they were not much more than the XS parts either.
There was a thread on here more than a year ago with that '58 Enfield, or whatever year, and it had a XS engine. The guy said it was a real, original bike, low miles, ect. By the time he had to take it off, for fausely advertising that is was all original '58 Enfield, it was up to several thousands of dollars and like 20 bids.
And it's true, people set those crazy prices. If sellers know they can get those prices, then they will stay high.