What is the value of a smith and Wesson 3931?

Trying to put a value on anything with just the limited information provided is hard to do. Value depends on a combination of many factors. You have only provided the most generic description. Without a detailed description to include, finish, barrel length, stock material, sights, box, original paperwork and any accessories, only a general estimate can be given. You could be in the 100- 600 USD price range more or less.