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.
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.
* 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.
* Hybrid HDDs have been around for years.