"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.

Yes, it means 20 dollars for each item.

I would translate as 20 bucks a go. 20 buck a pop might relate to a ride or other experience, or an actual item being purchased. Pop is literally to burst - as in to pop a bubble.

Thank you

"A pop" means "per item". It's an informal phrase that's used casually. I believe it's commonly used in American English.