Not sure about "plummet"; looks quite steady descent from May onwards to me. I would assume that last winters depression and possible production issues have been coped with, and the sales for DDR3 are increasing. All that tends to drive the prices down.
As for DDR2, it starts to be a legacy product. Legacy memory types do not get price cuts. It is and has always been dirty expensive to upgrade old chips (compared to the current mainstream model).