"A pop" isn't one definable thing, it's just whatever the person is talking about in this context. Saying "20 bucks a pop" is just another (very casual) way to say "They're $20 each."
For example, I could say, "I'm selling albums, 20 bucks a pop." I'm saying that each album costs $20. Or you might hear, "Those lightbulbs are 5 bucks a pop." Then that person is saying each lightbulb is $5. Point is, you can use it for pretty much anything.