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.
I've built boxes for computationally intensive workloads that use these PCIe SSD cards. The IO is STUPID fast, considering you're bypassing the SATA chipset and are limited only by PCIe speeds.
You don't get a large amount of space (I believe the cards I installed were only 64 or 128GB, but this was ~4 years ago), but for small datasets that you need extremely fast access to, they get the job done.
We have a few 480GB ones, and yes, they are stupid fast - we're getting 1GB/sec writes easily. You can get multiple TB now if you can afford it (can't justify it for our uses...)
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.
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.
The Cyrus IMAP server has a "split metadata" mode which supports this configuration. Very nice if you are running a mail server of any significant size.
* VMWare uses this as the root of their vSAN technology.
* Nimble and other storage appliance manufacturers use a bank of SSDs to cache frequently-read data, and have layers of fast- and slow-HDDs to store data relative to its access frequency.
