Tough to flush to spinning disk after your power goes down. So you'd have to do it continuously, which would remove most of the performance gains.

You include a battery in the package. You only need enough power to spin up the disk, flush from your RAM cache, and then power down. In fact, the DDR3 Disk Drive could include all three, RAM+Battery+Disk. Say, 64 GB RAM in DDR3 PC3-10600 1024Meg x 64 ( Crucial Part #: CT2KIT102464BA1339) for $600, 2.5" 80 Seagate ST980815A for $44, and a 10.8v, 4800mah (overkill for flushing, but they are cheap) PA3534U-1BRS battery for $19.81.

Add a charging element for battery $3.75, a controller for $5, Ram Sockets board for $4.25, Disk Interface connector for $2.00, case for $7.00, assembly for $6.50, assorted screws/packaging for $1.00.

You could sell a 64 GB DDR3 disk with backup disk for $693.5, 87% of which would be the cost of the memory itself - large such systems would be mostly the cost of the memory, as the other components (disk) don't increase in cost much, and except for the ram sockets, none of the others increase in cost at all.

Battery would need to be swapped out every four years, or so, but that would only be $20 cost each time.

Looks good, but people will forget to change the batteries and then fill the forums with complains when they lost all their data.

You keep a capacitor charged with enough energy to write the DRAM back to flash after the power goes out.