If you're building something on the cheap, then you could get an even cheaper setup with a 4port SATA motherboad, and an add 4port SATA PCI / PCI-e card.
Some of those eSATA enclosures will use 1x SATA port per HDD, and some even work on USB which is even worse.
If you can run each HDD on it's own port, you'll get optimal performance. For a cheap in-office storage, I use these:
http://www.google.com/products/catalog?hl=en&safe=off&q=icy+dock&cid=7046738176421693906&sa=title#p
It's basically a hot-swap cage, but I need to manually tell the OS that the drive was removed, since it doesn't run on a server back-plane. But it does the job as far as cheap storage goes :)
Put 4x 2TB HDD's in there and you have 4TB storage on RAID10