For me, any time I see a classic SIG in very good condition $500 dollars means it's coming home. I wouldn't pay that for a second of most models but would for a P220, or a P226.
Unfortunately buying and selling are two different things. IMHO it is worth something like $550, you will be lucky to get $400 and $350 gets closer if you are talking to a dealer
I would get over to The Sig Forum and politely ask around to get a better feel for your geographic area.