Original Model 12's (as opposed to the Japanese made replicas) in 20 gauge are getting harder to find. A field grade example recently sold at the Bud Haynes auction for $650 plus buyer's premium and tax. As Grouse Man pointed out, condition, grade and features will affect the price more than anything else, but all things being equal the 20 gauge carries a premium over a 12 or 16 gauge example.
Sharptail