THIS.
if you celebrate the holidays coming up, perhaps you can ask for a thermostat for Christmas?
What is best for your animals isn't always best for your wallet. There are heavy decisions you have to weigh when owning animals who rely on their environments and you for everything. And this may come off ass rude (which I don't mean for it to be) but i am wondering why you are getting an animal you can't afford to care for? If you don't have money for a thermostat.. do you have money for vet visits? Perhaps a payment plan would be a better option if its available? It would give you more time to prepare for your animal. Building cages is usually way less expensive than buying them. Thats a great way to cut costs.
I tried cheap "thermostats" i replaced mine in less than 3 weeks because it wasn't safe for my snake. The zoomedReptitemp500R i had would leave the INSIDE of my enclosures hot spot at 114 degrees F and that was when it was regulating! I sold that thing and got a Johnson Controls and haven't looked back since.
I understand you're looking for an inexpensive alternative but there are other areas where you can cut corners. (caging sometimes, hides, decor, etc) Heating is not one of them. IF you absolutely need to go the cutting corners route, don't use flexwatt. Everything I have ever read and seen about flex says it NEEDS a reliable thermostat. Flexwatt was not originally manufactured for reptiles, it was made as underfloor heating if memory serves me correctly.