I just did a quick check of the difference in electrical consumption: 8 melamine cages each with 1 UTH rated at 8 Watts = 64 Watts. To heat them with lights would require at least 25Watts per bulb, or 200 Watts total. Just for those cages alone, I'm using less than 1/3 the electricity with the UTHs. If I were to use lights on the screen top cages, they'd probably need 60 Watt bulbs since the heat tends to rise, so it takes more energy to 'force' the heat down to where the geckos are. That would mean 12 60 Watt bulbs, 720 Watts total instead of the 96 Watts it takes for the 12 UTHs.
That's just part of the collection. Don't forget, a lot of animals can easily take a night drop to room temperature, so you don't have to run any lights or UTHs at night. If you live in town there's enough light coming in through the windows from street lights for them to see what's going on (in the wild they don't even have the luxury of lights, it's moon and starlight, and quite often not even that if it's overcast or a new moon).
Oh... don't forget to use timers. The initial outlay might be a few more dollars than you'd like, but it will all pay for itself in a matter of a few months.