B&H Photo-Video offers the Drobo S 5-Bay USB 3.0 / FireWire 800 / eSATA Storage Array Enclosure, model no. DRDR4A21, for $508 with free shipping. Outside the mention below, that's the lowest total price we could find by $62. This enclosure can hold up to five serial ATA hard drives for up to 20TB total. It features BeyondRAID technology, which chooses the correct protection algorithm based on data availability needs and allows switching from single to dual disk redundancy with a single click.

A close price: Adorama offers it for $509.95 with free shipping.