Maybe it isn't justified for this purpose, but SAS doesn't have to be exponentially more expensive.
First, why bother? Well, eSATA sucks compared to SAS. SAS is faster, better error reporting, more options, better drives, better cables, better controllers, and far more expandable in a huge way.
Seagate has 7200rpm ES2 SAS drives that are only a few bucks more than SATA drives. If neccessary, you can add a couple of 15k drives for intensive tasks (video editing?) alongside 7.2k drives for archival. All that without slowing the bus to SATA speeds (you can mix SAS & SATA, but it slows the bus to SATA speed). It can be a huge bargain.
To the grandparent post: just buy the 4U supermicro chassis (plus JBOD power control board) that has a backplane with an expander (or two). These allow you to stack up and span out as far as your controller/bus can run.
You could also just throw a motherboard in it and run OpenFilter or Nexenta to have a real storage server that does FTP, NFS, CIFS & iSCSI. Plus snapshots and such.