Extreme newbie wanna-be here....

[gaitedappy]

I would personally go to the expo and talk to bearded dragon and ball python breeders alike to get there advice on your situation. Plus, it would be good for you to get the opportunity to handle a couple specimens of each species. As far as the enclosure goes, it really depends on personal preference, I use glass set ups for both of my snakes, but I also live on Southern California where it's fairly easy to keep my humidity where it needs to be with minimal effort, that being said, I do prefer the PVC enclosures, but they have not been in my price range. As the money goes, if you purchase from a breeder at the expo (the way I would suggest for you), you can keep total cost down fairly easily. When I bought my baby in August at the super show, I think I spent a total of $80 on her, the enclosure, a hide and a bowl (I already had a temporary heat lamp at home and bedding as well), then spent a little bit more getting another hide and some decor the next day to make her more secure, you should be able to keep your set up in your price range fairly easy. Substrate is again, a personal preference as long as you choose one that is safe for your snake (I personally use coconut fiber). As far as light goes, you don't really need it for bps, just a way to regulate the heat, my snakes just have the light in my room from my lamp and the window.
I think that if you go and talk to the vendors and breeders at the expo, you should have more luck answering questions and getting a feel for everything than you would at Petco or the store with the unclean enclosures you mentioned. This however, is just my opinion, and I'm sure you will get much more helpful advice from more experienced keepers here soon.