Yeah I've got the Mega CD. It's certainly harder to get hold of than a PC/Amiga copy, but it's not in the same league as the FM-Towns versions or anything. I payed around $25 I think, and I wouldn't expect to get more than $40 if I sold it today - I'm no expert of course, I could be very wrong.
In terms of finding original boxes, the games aren't hard to come across it's just a pain to find decent quality boxes - they're going on twenty years old now, some of them. For example, my Zak McKracken
box has a pretty big rip on the side... it's regretable, but what can you do?
Anyway, as far as I can remember (I don't have it here) the Mega CD is a box roughly the same size as a Megadrive box, but it's paper instead of plastic and is about as thick as a DVD case. Inside is an instruction manual and a CD case with a copy of the game in. The CD is different to the other CD versions of Monkey Island
, it has two swords on or a monkey or something.
As for playing the game, you need a SEGA emulator and a ROM to kickstart the ability to play CD games, I don't think it's hard to find one. The gameplay is essentially the same as the other versions, there are no major differences except the controls, which obviously work without a mouse.
I don't know why I wrote all that, I just remembered this site.