If it is a newer game, used but in excellent condition, it will still be worth less than a NIB. The only exception being valuable mods like color DMD, well done powder coating, PDI or invisi glass, etc. And those only add what they cost.

If it is an older game and in "like new" condition with desirable mods, I think it is worth what a similarly good new game in similar condition is worth. So a Twilight Zone in excellent condition with remote battery, LEDs, and color DMD is worth slightly less than a NIB JJPOTC.

It seems highly likely that the OP is asking because he is considering selling a game he considers to be "mint". It would help us help you if you told us more about the game you are selling.