When considering ammo costs, I suspect most will consider a per box (of 25) cost. Not the total costs because of differing quantities shot. When reloading shot shells the costs of wads, primers and powder are fairly constant for all gauges. It's the varying shot charges that are the major variable.
From a 25 lb (400 oz) bag of shot you can load:
355 (14.2 boxes) 1-1/8 oz 12-ga loads,
457 (18.3 boxes) 7/8 oz 20-ga loads,
522 (21.3 boxes) 3/4 oz 28-ga loads, or
800 (32 boxes) 1/2 oz .410-bore.
Assuming lead shot retails at $30 a bag, shot costs for reloading are:
$2.11/box for 1-1/8 oz 12-ga,
$1.64/box for 7/8 oz 20-ga,
$1.41/box for 3/4 oz 28-ga, and
$0.94/box for 1/2 oz .410-bore.
Most serious reloaders buy in bulk and pay about 2/3 of retail for shot. This drops the per box shot prices to: $1.40, $1.09, $0.94 and $0.63