Currently SSDs are eight to ten times more expensive per GB than harddrives, so the cost of making an SSD is going to have to drop by more than half.
The lower the price of SSDs go, the more people will use them instead of a HDD. This should lead to better economies of scale, reducing the price of SSDs further (unless we hit a problem with supply, real or manufactured).
Conversely, as demand for SSDs increases, demand for HDDs drops. Initially this would result in reduced prices, but it will lead to fewer and fewer people making them, and the price eventually rising.
So, we are likely to hit a critical point where SDDs wipe out HDD sales before they hit the crossover point, and even that crossover point could well be at a higher price than we currently pay for HDDs.