It's pretty simple: just search for the vitamin/mineral/supplement you want, and it displays them all ranked by the most amount of that supplement per dollar.

Multivitamins and Omega 3s work a little different, and protein powders are grams of protein per dollar, but that's the gist. Anyway, the affiliate link isn't even set up yet, but maybe some people could find this useful in their personal lives. Open to feedback!