a lot of variables. year & condition being 2 big ones. colts been making saa revolvers (peacemakers) for over 100 years.
most reputable gun-shops will estimate value (for insurance purposes) for free, or a small fee. get estemitaes from a couple, not all guys know what they are talking about just because they sell a few guns. look for a guy who sells used guns and not just new ones.
you need to have your gun value specifically listed on your renters or homeowners insurance anyway as most policy's I've seen only give you $2-3k limit on gun value unless you tell them to raise it. for instance I have a $100k policy for personal property but it only had the 2k limit for guns, so I had to pay a couple bucks a year to get that raised. you don't have to give the insurance company a list of your guns, just pay to raise the limits, and have a list with numbers and estimated value, thats in a safe place in case of fire, theft, or whatever.
also get a copie of 'the Shooters Bible', lots of useful info on many guns in there.