Well, MS' drives are probably more than that ($20-30), to be fair. Those Newegg 20GB drives are 1.8" hard drives - where 20GB is a 'standard' size. On a 2.5" drive like the 360 uses, I would expect it to cost more. Not Betan's numbers, but more than an equivalant 1.8" 20GB.
Even 40Gb 2.5 drives go in the neighborhood of $50-$100 on new egg. There is a mail in rebate for one model that prices it at $36.00
If NewEgg can sale 2.5" 40Gb harddrives one unit at a time for $50-100 dollars then MS should be able purchase millions of 2.5" harddrive for a fraction of that price. 10 million HDD units over 2.5 years require an average production rate of ~11,000 HDDs a day, 7 days a week.