I bought my 20 gauge for $420. The 12 gauge version often fetches a higher price. Funny thing is that the non AOW Kampers can often be had for dirt cheap.
Here is a 12 gauge model for sale.
Kimel Kamper 12 Ga. AOW Single Shot
The 12 gauge model typically has the barrel cut down more than the 20 gauge to lower velocity because otherwise the recoil would be uncontrollable.