Checking completed and active auctions on eBay might be the best way. Tomart does publish a magazine on Disneyana and frequently includes a Disney pin price guide. Pin values do fluctuate quite a bit, but older pins are usually a bit more stable in value.
Pinpics.com is a great directory of Disney pins, but the only values they list on there would be the original retail prices. However, many traders use the members trading vs. members wanting numbers as a judge of the demand of a pin.