I buy flake food by the 5gal bin to save $. Also, I buy commercial feed pellets for catfish and salmon. They're higher in protein than most LFS brands and run about $17 for 50lbs from a feed and grain store. For herbivores, I never buy fish food, I buy rabbit pellets at $15 for a 50lb bag. Rabbit pellets are higher in veg protein than fish pellets and 1/20th the cost.
For carnivores, well...I raise crayfish for the most part. I also buy bait fish and catch other fish to feed live and/or cut up. With over 100 tanks my utility bill is about $100/month. I have 2 solar panels (no storage batteries) running and am putting money aside to get another. I also run most of my tanks off of 1 blower to commercial-size sponge filters (good to 400gal @ .25cfm). I dropped a grand on attic and wall insulation and 4 grand on replacement double-insulated windows. Now, a small furnace/heat pump keeps the temp level at 80F in my 7500+ sq/ft fish building.
There are other methods to save money but, gotta go feed the fish now.