Eventually, they'll probably primarily replace HDDs...though how long that takes is going to be dependent on several factors (drive life, whether they can make an efficient controller to handle that much data, etc). When that happens, the prices will drop much like HDDs have dropped (excepting the flood). I mean, in 1997 a 6GB HDD cost me about $200...now that's 3TB territory, a 500 fold decrease in price per gigabyte. They won't reach parity with HDD costs for some time, but I would not be surprised at all to see 256GB drives at the $100 mark within 3 years.
I mean, look at it now...we're paying about $1.25/GB on average for non-cutting edge drives, and about $1.60/GB for the newest tech, while less than 2 years ago, SSDs were around $3.00/GB.
I was lucky to get my M4 for $150...gone back up $20 now, but still not too bad.