#1: Yes you can keep your bp in a 40gal. If it is a screen top lid you will need a large shallow water bowl and you will need to cover most or all of the screen top lid with plexi glass or bath towels to hold in the humidity, expecially if you are using a heat lamp rather than an under the tank heating pad.
#2: Yes you may be able to find bp's at that price but remember you get what you pay for. If you want a healthy ball under $50 get a corn snake. Pretty much any bp you see under lets say $70 that is not from a reputable breeder is probably CH or farmed or WC. And one question for you: If you can only spend $50 on a snake, where are you going to get the money for heating supplies, cage furniture, thermostat, digi thermometer and the cage?
And finally, I am not trying to discourage you, just making sure you understand the cost to getting a healthy BP and the cost of getting proper equipment to house and keep the snake healthy.
P.S. Bet you can't stop at just one BP once you get it! I was going to get one bp "just because I like them." Yeah right 6 more two months later... Is there a BP anonymous? Not that I want to give up the addiction but... my fridge is empty! lol j/k -Joe
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0.1 Green Iguana (Sisco)
0.1 Rose Hair T (Sweets)
1.1.4 Hairless Rats (Socrates, The Three Stooges and Left Eye)
3.2 Ball Pythons (Stratos, She-Ra, Skeletor(0.1 oops on the name!), Sy-Cloone, Stinkor)
1.0 Striped Cal. King (Snuggles)