I'm not sure if Jan Still's Luger forum is still up and running (he passed a while ago?) but you could give that a try. A search on a firearms auction site could give you an idea of what comparables have sold for--the key is to look for "sold" or "completed" auctions, not what people are asking for their pistols--I've seen pistols offered for sale at $XXXX for several years on some sites.
Good luck!
Good luck!