As said, I think the one size fits all answer is whatever the buyer is willing to pay for them. In my experience I've found it's always better to remove most of the mods, toppers, even the color DMD and sell them separately. Toppers especially seem to be give away items. If any mod makes the difference on selling a game for big money or not, that mod is probably worth quite a bit as a stand alone item.
I've had a couple games listed and by the time I did the math on the mods value used(removed), it came out to more than I was selling the machine for with them included. Both times, I removed the mods, sold the game, then sold the mods and came out ahead.
