"NAND" flash memory is a type of storage technology that doesn't require power to retain data. This is why NAND flash memory chips are used in USB flash drives and many electrical items such as smartphones and tablets. NAND flash memory chips are manufactured by just a few hi-tech giants like Samsung, Intel and Hynix in a variety of capacities and levels of performance.
The price of NAND flash memory changes daily because it's a commodity just like coffee or gold, and it fluctuates based on supply and demand. Samsung, Intel, Hynix and Toshiba control production to ensure the global market is neither oversupplied or suffering significant shortage. Demand for smartphones and mobiles devices has been at an all time high and NAND prices have reflected this over time.
Well known companies such as Apple, Sony and Nokia have the ability to influence NAND flash prices too according to their powerful demand. This is why you see price fluctuation when there's a new iPhone or other big phone releases. iPromo USB flash drive prices change according to the NAND flash prices, because the NAND flash chip is the most significant component in a USB Flash drive from a cost perspective.
SSD solid state drives have also affected NAND flash memory prices recently. SSDs became a major component in the computer hardware industry in the last 4 years because they are 7 times faster than mechanical hard drives. NAND flash memory is an integral part of SSDs, thus increasing demand. In the near future, SSDs will replace the current hard drives on the market when cost begins to drop as manufacturing technology improves. As a result we will also see a stabilization of NAND flash memory prices.
As NAND flash is a traded commodity it is hard to predict how the price will change in the immediate future.
Take a look at the following article from ZDNet: NAND flash shortage leads to higher SSD prices, fewer drive shipments