What is the value of Richland Arms Co 20 gauge San Marco model 747?

Trying to put a value on anything with the limited information you provide is just about impossible. Value depends on a combination of many factors. You have only providided the most generic description. Without a detailed description to include, finish, barrel length, choke, stock material, sights, box, origiinal paperwork and any accessories, only a general estimate can be given. This weapon could be in the 10-300 USD price range more or less.