Most things are pretty obvious; Color DMD adds it's price, your stupid glued on topper adds nothing.
It only get's confusing when you come to things boards, which I've run into a few times.
If you've just bought new boards to put in your machine, they add no value to me at all. It's weird, because you just spend $400 to get your game working, but now it's juts a working game, which is what I planned on buying anyway.