No offense, but if you can't afford a thermostat you probably shouldn't be keeping pets. Any animal can get sick and require a visit to the vet. At the low end you are look at $50+ and in some cases several hundred dollars. Maybe you should think about putting some money into a savings account a little at a time in case one day something does happen.

As far as heating, you can get a decent thermostat for less than $30 or so and a section of flexwatt already wired for about $10. At the very least you can buy a $10 rheostat(light dimmer) from walmart that will be acceptable if your room temperature doesn't fluctuate more than 5-8 degrees. On my tank I use a UTH I bought before I knew about flexwatt and it doesn't provide enough heat for night. The tstat controls it during the day to regulate the temps, but at night I turn on a night heat lamp(low wattage purple) to compensate.

As for humidity you can cover about 2/3 of the screen top in foil and adjust your water bowl size. This works for me to keep around 55% humidity all the time. During shed I mist once every day or 2 to keep it at 70%.

For substrate aspen is cheap, dust free, and looks decent.

Keep in mind, without a way to regulate your temps you could bake your snake. If your A/C decides to quit during the summer while no one is around your lamp could easily reach into the 100s. All it takes is one time to kill your snakes. So maybe party a little less, hold out on those new clothes, or whatever you can get by without to save a little cash for your living animals. It's not a large monthy investment, just once unless something breaks.