I think they can be found for £500 because most people don't know what they have. £500 isn't the wrong price either,.
However, once in the hands of a collector then they are not going to let them go unless for something better. Better condition, better example, or more suited to their collection. Between collectors it isn't the price but what adds to the adventure.
Its anyones guess to price on any one day.

The limit is what could be had for the same kind of cash?? A new tuned action, reblue, and high grade stock with chequering, then what price that custom? What premium because its from a 1980's custom stable?