In the big picture, are the drives that expensive? Your monthly bandwidth bill is going to come close.
Measuring latency (vs seek time) is definitely a better metric. But is a 5 second (the horror!) delay to watch a 40 minute video that bad for user experience?
A better example may have been streaming music, where we already know last.fm got a big win from using SSDs.
The drive themselves might be an extra 10 or 20% of the cost of the machine. They come with other factors as well. 5x the disks means more power draw and cooling. It probably also means you need a 2U chassis instead of pizza-box 1U, which means your datacenter rent doubles. (It may be possible to fit 10TB into a 1U. I've never tried it, but I suspect you'll have space / power / heat problems.)
Measuring latency (vs seek time) is definitely a better metric. But is a 5 second (the horror!) delay to watch a 40 minute video that bad for user experience?
A better example may have been streaming music, where we already know last.fm got a big win from using SSDs.