OP, you cannot beat the 12ga factory prices, however, that is not what you asked.
I reloaded several hundred 20ga shells a year for about 4 years when I was a kid. Friend had the loader and components at first, then I bought the next round. We shot grouse, rabbit, pheasant, and raccoons ALL the time. Actually, at sixteen, I shot my first deer with a slug round I had made.
I believe that you can and will save money loading your own. I have 2 Lee Loaders now, one in 12ga and one in 20ga. I also have two 600 Mec Jr's. One set up for 20ga and the other for .410. I reload everything! Wife allows me to use a portable bench I made in the living room and loading shotshells is fairly safe to do while "attending" a movie with her and the kids.