If you've got "I Need You" in a slipcase, it's worth the most, not as many of them went out. I've seen the other slipcases (empty) offered anywhere from €15-€25... so maybe €30 for "I Need You." With CD's? Not much more, the discs are pretty easy to find.
Incidentally, I need the cases for "Bottle Living" & "Dirty Sticky Floors"..... was about to trade on depmod but hey, if you're looking to get rid of them.... well, PM me.