People are doing that. If you see those PCIe "SSDs", most of them are effectively multiple SSD's + controller on a card, combined with either software or hardware RAID-0... E.g. some of the OCZ cards at least show up as 4 individual units under Linux, while the Windows drivers at least by default will show it as one device.

But yes, it is expensive if you measure per GB. If your dataset is small, though, and you are more interested in cost per IO operation, they can be very cheap.

We run 8 SSD RAID 10 for all our database nodes. However, you still need the same metric ton of RAM you'd need before. However, if you have an I/O heavy load [which almost all databases do] you can cut down on total number of machines compared to SATA.

RAID SSD has the same latency as a single SSD. This setup will help where the bottleneck is I/O throughput.

SSD prices are about as high as HDDs have traditionally been. The difference is now the bottom is falling out of the HDD market and you can get 3-5 2TB HDDs for as much as a 256GB SSD. So lots of devices and appliances use a single SSD for read- and write-caching, backed by a bank of high-capacity HDDs.