The "plug-and-play" options available from Johnson Controls and Ranco range from $50-$100 depending on the source. I second meadmazer's suggestion of the STC1000. I purchased a similar model directly from China except mine reads in Fahrenheit (Total price after shipping was $14.66). Both require a bit of wiring, but are worth the savings.
I replaced the existing temperature controller in the front of my unit with my new temperature controller like this:
(The numbers correspond to the numbers on terminals of the new controller)
Not intricate, very easy to do, which was your goal.