Since you are on a tight budget, here are some tips for being cheap.
Check out the dollar store. Usually they have a lot of fake plants and flowers and stuff that you could use for decoration. Also you can use opaque plastic bowls as hides. Just flip them over and cut a small opening in the side and tada! Perfect ball python hides.
If you don't want to keep buying snake bedding (I don't blame you) a cheap alternative is paper towels or newspaper. You can also use the aspen made for small animals instead of snakes. It is about $5 for a HUGE bag. It will last you forever with one snake.
The best thermometer hygrometer combo is actually not even made for reptiles. It is the acurite weather station. It is sold at home depot, lowes, and walmart for around $12. That will be your best investment. This unit will measure your hot side, cool side, and humidity!
It might be worth your while to cover the back and sides with dark construction paper or foam core board. This will help the snake feel more safe and it will also help keep the temps more stable.
At some point when you get more money (like $50) I would invest in a UTH with a thermostat.