Look at the price of 512MB ram when the 360 came out, that should give you an idea. (~Feb 2005 - Mushkin PC2700 CL2 512-MB = $101) Compound this with the fact that the 360 shipped with 4x the amount of ram of the original Xbox, and was released 4 years later. The successor to the 360 will be released ~8 years after the 360's original release date.
I don't remember 512 megs of RAM costing $100 in 2005. Where'd you get that figure?