Well I got the insulation for free, so pretty much just the electronics, 50$
BUT, I went the easy route and purchased a thermostat already built for a CPU fan, and the power supply that comes with it. You could easily knock 50$ in half by purchasing a household thermostat and doing the wiring yourself. It seems fairly easy, I just didn't want to go that route.
I would say if you were to purchase the insulation and the the electronics yourself, I'm saying around 60$-70$