You cannot legally buy them from retailers, as far as I know, but I designed and built mine for around $2,700 total. This includes one fully-automatic AA-12, one magazine, and a set of iron sights I...
Do you need a soft case or hard case? Prices can range from $10-$30 for a soft case & from $15-$200 for a hard case. Take a trip down to the local sporting goods-gun shop.