The 1" x 12" being 3/4" x 10 9/16" is actually standard. Sorry if you were disappointed, but it's how the business operates.
As far as the economy of it all works out, there is a big difference in the type of wood you get. A 1x12x8 "Select Pine" Board will run me about $24. However, a 1x12x8 "Southern Yellow Pine" Board will run me about $9. Not that I buy in 8' boards, but if I did . . .
All in all, when I last ran my numbers, I made a super for about $5.50. I only run mediums, so that would calculate into things, and that doesn't include labor or the wear and tear on my powertools, but still it's at worst $3 a box cheaper than I could get anywhere else. That's like buy two get one free. Yes please, lol.
Sometimes I've been known to take Furring strips, glue a few together and make a few boxes out of that. It makes it a little more challenging, and they don't always last as long, but I can make one of those for about $3 if I play my cards right.
In the end, it's worth the effort for me. And that's not adding into it lumber mill prices (which I will be hitting up soon).