I worked in the kitchen at the prison I was in and made $17 a week. Worked everyday from 9AM-7PM. And that was one of the highest paid jobs in the prison. Doesn't mean people don't make money though. The guy that was running gambling pools would send home like $800/week to his family. He had been doing that for 6 years by the time I got there. So people involved in things like drugs and cell phones make thousands, literally.

I got money from my parents so I wasn't too uncomfortable, had a TV and got books sent in, that kinda stuff. At the place I was at you could spend $120/week.

As for protein I just bought pouches of tuna and ate maple and brown sugar oatmeal with peanut butter. You don't have many options in prison, but you won't get big on the prison diet alone, just isn't enough. Also another benefit of working the kitchen.