I only spend 10 to 20 dollars per month, sometimes less. The puffers eat a lot for their size but food isn't that expensive, I get snails for free and other stuff is about 5 dollars and usually lasts more than a month.
Less then $10. I get snails for free, and bought the big pack of frozen brine shrimp, which I am still working on. Give bloodworms every once in a while...tried krill, but they didn't go for that.