well sadly there are no cheap cooling thermostats..
Not sure what you consider cheap, but I come to the pepper growing forums after having spent 10 years on the saltwater reef tank forums. I am recycling alot of my old reef tank equipment to be used for my veggie garden / pepper grow this year (ambitious first timer, LOL).
Anyways, we used a controller, the Ranco Dual Stage controller, for controlling both the heater and chillers in our aquariums. With a 300g aquarium, I had a 1/4hp chiller and a 2000w heater, running both at the same time would be just pouring money down the drain. But I needed to maintain an even temp, +/- 1-2 degrees at MOST. The Ranco is like $100 new, which is expensive, but you can find them used on many aquarium forums for $30-$50.. They have a ceiling and a floor limit, ie, ABOVE 82, turn on A, and BELOW 80 turn on B (A and B being 3 prong outlets capable of handling the wattage of the heater or chiller). They were pretty darn accurate for the tanks, and if you gave yourself 80-85 as a target range, I bet it would avg 82/83 all the time without venting unnecessarily or running the heater while venting.
Since my basement avgs 70, and under MH's it can easily get up to 80, I haven't played with the controller for maintaining air temps, but I don't see why it wouldn't work and I'll definitely be playing with mine in a hoophouse I have outside later this spring (yes, they are waterproof too which is a bonus LOL)
