for a 300g with mh's and VHO's
electric ~$80 (1600w lights, return pump, cacl reactor, 2 powerheads, 1 closed loop pump)
salt $14 (20% per month or 60g or 1/3 of a box of io.)
food $75 (all frozen)
light bulbs $20 (assume change each once per year)
media $20 (GFO, calcium Co2, rodi)
total $209
this does not count the effect all the heat has on the AC bill which I believe to be about $100/mo. Also doesn't count water to run the RODI which i don't know what it costs since mine is free.
plus any repairs/upgrades/fish/corals/additives
if the tank were run with more efficient LED's you could probably drop the cost per month by $80 on direct costs and another $100 in lower AC bill