Like everything else in terms of collectibility value, the starting point is condition. Imo, whereas the original Winchester Model 70 has been rightly dubbed "the rifleman's rifle", the Model 12 could be called "the shotgunner's shotgun". Both were (and many would argue still are) time proven icons that all other rifles and shotguns were (are?) compared with to determine if they measured up to the "Gold Standards" of the day.
When it comes to all the important attributes a pump shotgun might offer in terms of material, workmanship, design and handling qualities, the Model 12 is hard to beat. Imo, only the Remington Model 31, the Ithaca Model 37 and (maybe) the Browning BPS come close.