This is already playing out. SSDs are cheaper than hard drives at 120GB, and that crossover point is moving upward. Sub-500GB hard drives will probably disappear from the market next year, now that 240GB SSDs are dropping below $30.

SSD controllers are cheaper than the fixed costs of hard drive motors, actuators and clean-room assembly. Adding an extra platter gets you more incremental GB per dollar, but getting that first platter working is much more expensive than the minimum viable SSD.