Hmmmmm from personal experience seen.....
Lot of first timers have severe anxiety once in and can't/refuse to eat. This gets you sent to the doctor inside. They will check you off for meal replacement drinks (ensure, or whatever they got in there). Now when you got that to drink, some guy is gonna say "give it to me" and this nervous wreck will hand it over. So there's some extra protein/cals.
Prisoners who work in the kitchen steal/eat while the guard isn't looking. These delicasies can be so delicious as a tortilla wrap with peanut butter, ramen noodle, and tuna all stuffed as much as possible and scarfed as quickly as can be.
Rec drugs are easier to get inside than on the street. Everyone knows who deals and they will sell to anyone. If you turn into a rat it's easy to find you and be dealt with. Just cuz a prisoner makes no money doesn't mean their momma/wife/whatever isn't paying someone on the outside for what you do inside.
Extortion works well inside. You may have to pay someone for protection. A measly $2 a day for him to make sure you live relative stress free. He will do this for many people. $60 x ? people a month