Even the 24nm toggle NAND is dropping in price as well.
Crucial are also bound to release a new SSD soon, the M4 is getting a bit long in the tooth. The new SSD will most likely be based on the Marvell 88SS9187 controller, which is more than likely very similar silicon to the Everest 2, but probably not clocked as fast.
There will soon be 19nm toggle NAND, and 20nm ONFI NAND.
Take your pick as to why the Crucial M4 is dropping in price, it could be any one, or all of the reasons listed above.