How do they determine the value of a game?

I have a few questions about how the trade-ins work.

If I take a game to a local GameStop store, how do they determine the generalized value of one game and what factors come in?

If I were to find a game costing $20, could I trade in 2 used games valued at $10 and still get it even if it's a new copy?How do they determine the value of a game?They determine it on how new the game is how many trade ins the game has gotten and how many sales the game has or hasn't gottenHow do they determine the value of a game?yea