im going to do a writeup on the one i built for less then 10$ once i figure out how to post pics. i think a waterheater thermostat is about 10$ and i hear the temp has to drop about 10-12 deg befor it will kick the light back on (so i heard) i had a old fish tank heater layong around. i took it out of the glass test tube it was in and clipped of the heating element wires. i then used butt conectors and crimped on the 2 wires from the female end of a extention cord. i pluged my light into the extention cord and it works great. i have a 40 watt bulb that stays on all the time and a 100 watt wired to the thurmostat. most people say to just use a 40 watt bulb but im using more watts to make more heat as i have my incubator vented pretty good to keep the air inside fresh.
good luck to you.