Hi there,
I don't mean to be obtuse, but I find it odd that you don't like the prices charged by the specialists who provide invaluable help, advice and reproduction parts for our rare cars, but you do hope to get 5000 for a used hardtop.
Without these suppliers we would have a lot of trouble getting parts to keep our cars running.
I'd suggest the prices I can see for reproduction gear knobs on the sites you mention are entirely reasonable, and in keeping with cars valued in excess of £50,000.
Cheers, Brian (BS1319/1470)