My horses eat frozen. I have 4 erectus, they eat 2 cubes of hikari or PE mysis a day. A firefish and a couple of peppermint shrimp get the leftovers. I fast them one day every couple of weeks.
I give them Hikari Ocean Plankton every few days, too.
A flat pack of any of those three foods is about $5, and I think 3 of them will last me about 2-3 months. So, ~$7 a month?