Heaters are really not all that expensive to run. Tropical temperatures aren't that hot, for a start; a lot of newcomers are surprised how cool the water feels, its's only just luke warm for most fish. And most people's houses are warm enough that the heater, as daize says, isn't on for much time in every 24 hour period.
My eldest son reckons it costs about £2.50 a week to run all my tanks, but I
do have six running and two of those are over 200l with big externals (one is a 5'x2'x2'!), so I think that's alright really