Bill
I have quite a few Beolits, though as Martin points out, the 500 is an unusual member of the Beolit family being as it is a combined radio and intercom. Having never seen one for sale, it's hard to say, though I am sure there is some curiosity value attached to it. I guess the answer to the question is that it's worth what someone would be prepared to pay for it! If that's not helpful, I can tell you what I paid for some of the other Beolit models over the past year:
1960s Beolit 600 - around £80ish for a nice set;
1970s Beolit 600 - around £120ish for a nice set depending on the colour of the trim panels;
Beolit 800 - around £100, though that particular set was a bargain in my opinion.
Based on the above it might be fair to assume you might expect between £50-100???
John
President, Beomaster 8000 Appreciation Society