Having amortized the cost of equipment long ago, my economics are a little different from persons just starting. I can save very little by loading 12 gauge now. I can save a bit by loading 20's. There is still a lot of savings on 28 and .410.
Lead is the big cost item now, but you can save a bit by using 1 oz, 7/8 oz, or even 3/4 oz shot charges in the 12 ga. I also feel a lot less fatigued after shooting 4 rounds of Skeet with 3/4 oz handloads, than I do after firing 4 rounds of Skeet with Remington/Winchester/Federal promotionally priced factory Ounce-and-an-eighth loads.
Cases used to be a big cost item, everybody wanting to use Winchester AA's. Twelve and 20 gauge cases are essentially free if now you pick up other people's Remington empties, all of which load about as well as AA's (the exception being UMC, actually loaded overseas).
A lot of money can be saved by buying nock-off wads, but you have to buy in quantity to save. I have a huge inventory of Winchesters, but Claybuster & Downrange are two makers I like.
A good bit of money can be saved by using a propellant such as Promo and using Cheddite, Fiocchi, or Nobelsport primers.
Make your selections, then use this calculator:
http://10xshooters.com/calculators/ShotshellReloadingCost/
CDD