Prices were reaching an all time low for all sizes until a monsoon hit Thailand. Prices are hovering around 50-60 for a terabyte (1000GB) hard drive now the prices are closer to $140. And its not just Western Digital's hard-drives Segate and Samsung also have manufacturing facilities in Thailand that have been effected by the flood.
Best solution? Buy and SSD drive instead as the prices are almost the same now :)
Hopefully in a few months the prices go back down.