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.
