They won't drop in price until they have a larger model out, which isn't going to be any time soon thanks to the 2TB MFT limitation (you can't boot from a volume bigger than 2TB, so it's unlikely the manufacturers will bother making them becasue of the inevitable customer support headache)
What you might see is a drop in price for the old models when they move up to 666GB or 1TB platters, but that's going to take a while and they'll probably just phase out the old models entirely since a reduction in platters has cost benefits for them, but not a great deal of marketable performance improvement.
Considering how much more per GB they are, you're probably better off getting 1.5TB drives, and either building a networked file server or getting a better case.