£40 ($80) perhaps double if it is real museum quality, box and all.
Hard drives for A500's are not that sort after, users tend to just upgrade to an A600 or an A1200.
If you got two A500 zealots both at the same auction you might start a bidding war :-)
A good bit of advertising here on all the Amiga boards (in all languages) Amiga.org also Amibay(EAB), Amigaworld, LemonAmiga etc. etc. together with offering shipping anywhere in the world and ability to accept PayPal could improve the final price even more.