You're paying for engineering costs and materials, for the iMac and the Air mostly, but the others are pretty well made too. Getting a full i7 rig plus monitor to fit in a case less than half the thickness of your everyday LCD, and having it not overheat constantly is HARD.
Too much price for that engineers and materials, if not, see any hackintosh specs and prices.
An i7 on a airmac... are you sure?
