Everyone has different opinions on how they should be kept. Most people here will recommend you use a heat pad for BPs so they have belly heat. You can setup a cage pretty cheap or you can make some nice cages that will cost more money. You have to monitor your temps though, snakes need a certain temperature for proper digestion and they should really be given a heat gradient. Also a lot of heat devices get much hotter than we realize. Heat pads get especially hot which can burn the snake. The reason most people will also recommend heat pads because they help more with humidity. I personally keep my BPs in rubbermaid tubs with news paper bedding and Tupperware hides. It's cheap to setup and easy to maintain.
Just because your BP is well fed doesn't mean much. They need to have the proper environment or they will stop eating or worse get really sick.