Honestly it would be very cheap to build a decently insulated cage out of wood. Do you have any friends with wood working tools and a bit of experience? You don't have to be a novice to make a decent cage. Maybe less than $100 dollars.
Flexwatt is cheap when bought by the inch/foot. It's very easy to attach the cord. Just do a google search to see the best prices on the flexwatt and cord.
For a wood cage, I'd recommend putting a length of flexwatt on the inside (since the heat will not travel up through wood very well) affixed to the back wall, along the entire back.
This would be no different than a rack with back heat instead of belly heat. The enclosed wood cage would ensure that more heat and humidity is contained.
You just make sure you paint the entire thing with a non toxic, water proof, oil based paint long before you get your new ball python. Give it plenty of time to soak the wood and dry. Then finish adding your heat and whatever else you want.
Latches for the door are a few cents each.
I'd say well under $200 total for a good cage with everything you need.