I keep good track of all my shooting costs, I bought a lot of supplies just before the Obama induced shortages, and am just replenishing now.
To answer the Question, Yes it can be cheaper to reload 12ga, as long as you are cheap. The load i do is 7/8oz of Hummason lead, 18gr of Alliant Promo, CB Lightning wad, Fed 209A primer in either a Gun Club or STS hull. It costs me 5.47$ per box, or 6.18$ after tax here in Ontario. Factory Top Guns at Lawry's cost 76.78$ after taxes. Saving me a total of 14.98 per case, pretty big difference if you shoot a lot.
I used an online calculator as well as breaking it down myself, got the same cost result.