In 50+ years of fish keeping, I've bought only 2 tanks new, the rest used (that's about 20 tanks totaling about 1500+ gallons). So tank cost was minimal.
I could often pick up a number of 50 gal tanks free just driving around the university at the end of a semester.
Larger cost items, have been water pumps, even then maybe only $100+ each on-line for good ones
With about those 20 tanks, my energy bill in winter (Dec, Jan, Feb and Mar in WI) for my fish room (heat, lights, water changes) averaged about $400 per month, in summer about half that.