No offense intended to the op, but this question reminds me of an ad campaign Bell helmets ran years ago that said if you have a $10 head buy a $10 helmet.

Go ahead and spend the money to get a good reliable t-stat. If you're looking to save money, I would go with an on/off over a proportional. Johnson is a very good unit.
Reptile Basics is a great place to pick one up.
Good luck.