barrel comes off the action by pushing that pin at the top of the magazine in, and then rotating it about 270 degrees, then pulling the magazine up (or away from the trigger). Then twist the barrel 90 degrees and it's free. Prices on a model 12 range greatly but it sounds like yours with a standard lenth barrel, no rib would be 200-300 in the midwest. I bought one recently with great blueing but bad wood for 325. A good set of guts is worth a lot since the parts can be hard to come by, and die hard model 12 shooters such as myself will literally shoot them to death.
