But why the fusion drive over say a run of the mill SSD? The 500GB had a price crash last week at 120USD. Picked up a few for the office.

http://www.everymac.com/systems/apple/imac/imac-aluminum-tapered-edge-faq/how-to-upgrade-imac-hard-drive-aluminum-2012-2013.html writes:
"Storage Details

All of these systems have a 6 Gb/s Serial ATA (Revision 3.0) connector that is occupied by a hard drive in the default configurations -- a 2.5" hard drive in 21.5-Inch models and a 3.5" hard drive in 27-Inch models. An SSD can be installed in this hard drive space and some systems support a smaller PCIe-based "blade" SSD, as well.

Specifically, as first discovered via teardowns from iFixit and others, if the 21.5-Inch "Late 2012", "Early 2013", and "Late 2013" iMac models were configured with an SSD or a "Fusion Drive" at the time of purchase, the SSD module is connected via a dedicated PCIe connector and another SSD can be installed in its place. Unfortunately, if the system only was configured with a hard drive, the needed connector is not present on the motherboard and there is not a way to add a PCIe-based "blade" SSD afterwards."

That means to me, no fusion drive. Just old? tech SATA drive swaps.