There are actually a few computer fans out there that are speed controlled according to temperature as well... and they are a lot cheaper than 75 bucks. Something like
this would work nicely. After all that, all you'll need is a computer power supply and you'd be good to go... power supplies are nice, because you can basically run as many fans as you want on one (no matter no low the watt rating the power supply) and you can run additional things off of it... like an auto-topoff system, leds, etc.
I currently have no fans running since my tank is sitting in the basement of an airconditioned home and has a hard time going over 80F even in this weather, but if I wanted to, I could easily hook up fans to a computer power supply that I salvaged from an old 386.
-Richer