They're expensive, but you get what you pay for. 3.5" HDDs are made to work sat in a non-moving PC, and are sold in huge quantities - there's also a lot of price pressure.

2.5" HDDs are far more rugged, are sold into fewer applications and so are more expensive, and when you get to the top end (eg, IBM 60GB) there's very little competition - so they can sell them for what they like until another vendor starts shipping 60's.

Bear in mind that these drives typically go into top-of-the-range $5000 laptops.

Hugo