I converted an old Red Bull bar fridge into a incubator. It holds 6 clutches and was about $200 total for the fridge itself, the flexwatt tape, the fan and the proportional thermostat.
You can see the details of the conversion here: http://ball-pythons.net/forums/showt...ull-Incubator!
You can usually find old glass door fridges with busted compressors on criagslist for dirt cheap which are not hard to convert in a incubator.
The most expensive component (and the most important) is a good proportional thermostat. I recommend either the Reptile Basics VE-200 or the Herpstat 1 Basic.