If you load your own shells, expect 75 or so shells per pound of powder with 12 gauge. You won't find it cheap.
I get 50 loads per pound when loading 10 ga skeet loads. 4 drams with 1 1/2 oz of #9 in a modern AYA 10, what fun.
I shot some 12s at a sporting clays shoot Monday.
The other part of BP loading is good wad pressure. Plastic over powder wads melt. Modern plastic wads with the crush section cause BP to treat the shot cup as an obstruction. I have heard it can cause bulged or burst chambers. Others may have first hand accounts.

Joe