http://www.newegg.com/Product/Product.aspx?Item=N82E16822136387
prices as of 2/1/2010
250GB hard drive $49.99 (with a little modding and a lot of command line, you can upgrade the hard drive in your current xbox 360 enclosure
PS a 320GB 2.5 inch drive is $59.99
a 500GB 2.5 inch drive is $74.99
a 640GB 2.5 inch drive is $107.99
A 750GB 2.5 inch drive is $159.99
now on to 3.5 inch drives (microsoft could easily allow external drives via USB or even just add a link cable that can be connected to the xbox 360 drive bay, then connected to a bare hard drive that the user can just set on top of the console (it will be safe since either way, you wont be moving the console or knocking it around as if you do the console will most likely break before the hard drive does)
250GB 3.5 inch hard drive $44.99
320GB 3.5 inch hard drive $47.99
500GB 3.5 inch hard drive $54.99
640GB 3.5 inch hard drive $59.99
750GB 3.5 inch hard drive $69.99
1TB 3.5 inch hard drive $84.99
1.5TB 3.5 inch hard drive $109.99
2TB 3.5 inch hard drive $159.99
Microsoft's hard drive which is basically random brand hard drive inside of a cheap enclosure with a special adapter since the console it's self uses a propitiatory connector, cost $10 more than the largest single hard drive that money can buy