Shotgun forearms get lost while hunting and they get separated from guns during maintenance, etc. And a gun is pretty much useless without the forearm, so they absolutely have value.
Pricing depends upon condition, presence of engraving, rarity, and demand of course. Naturally, a small frame forearm iron for a 28 ga or .410 will be much harder to find, and more valuable than one for a more common # 2 frame 12 ga. The best way to get an idea of realistic pricing is to spend some time watching actual sales on Ebay
