It's a lot more than 10-20% for a lot of desktop users because Apple doesn't offer a middle of the road desktop without a monitor built in. Personally I already own two great LCDs, so when I went to pick one out, my options were the mini (which has only one video jack and is pretty weak by desktop standards, though small and silent and therefore great for casual use or probably as a media hub) or some ridiculously expensive beast of a Mac Pro. So the price jumped from like $650 to $2,8000.

If I could have gotten something with iMac-like specs but minus the monitor it still would have cost more than a Dell, I'm sure, but at least only by 10-20%.