A PID would be best, but this means getting at least a relay but really an SSR to utalize it to its full capacity (ebay may
have a cheaper option, but it's hard to beat Auber's PIDs... I tried... it's only their shipping that stinks). $52.50 for the PID shipped plus wires, SSR, heatsink, temp probe and some kind of project box. You're looking at $85+ (depending on where you source parts), but it can scale up to 240V if you want and would give you precise control.
But you said cheap. This would probably be cheaper: build a temperature controller similar to those used for ferm chambers (Similar to this: http://www.homebrewtalk.com/f51/ebay...-build-163849/
Since you're doing 120v that would work fine, with limitations. A PID is designed to 'dial in' a temp (pulsing variable output), a temp controller will be ON or OFF, and will (probably) overshoot. But you can adapt to that as you learn the system. Even a PID isn't perfectly precise.
Or you could use an old/new ronco/johnson controller (cheapest option if you have one)... new they're not cheaper than these other options though. I would have done this if I went 120V since I have a johnson controller.