Flash itself is dirt cheap. Making an SSD just requires a controller to handle the I/O, the wear levelling, and other functions. I imagine you could do it with a small, low-power CPU, a small amount of firmware space (on a separate, smaller flash unit?), some DRAM for caching, and energy storage to let the device flush the cache in the event of a power drop.
Fitting all of that into a standard hard disk form factor would probably be doable. I imagine the high costs on SSDs are mostly due to the development costs.