I use the Animal Plastics T8. (48"x24"x12" LxWxH) this is the ideal size for BP's in my opinion. It costs $150 + $45 for shipping to get the base model. You can have things like viewing lights and Flexwatt Belly heat added on as well.
Here is their website:
http://www.animalplastics.com/
I agree with Mike. Use the 40 gallon for now and save up for a PVC cage. Remember a BP can live for 20+ years so a cage like this is an investment that will last a long time.