50 bucks a pop is referring to a set of the answers to the math final. So, you pay 50, you get the answers.

"50bux a pop"! Means 50bux for each item or article. It is slang, for 50bux for each one, each go or each product for example. "To rent a surfboard is 20bux a pop"! It's 20bux to rent a surfboard!