MPG recently noticed that the cost of an iMac 64GB memory upgrade kit has risen from $698 to $799, a 14% price rise. Here is some insight from OWC / MacSales.com:
It's a perfect storm with sustained traditional demands, but with substantial growth in mobile consumption due to a doubling of both memory and flash per device carrying the year.
Tightness was already forming - add to that the new 3D flash geometries that are still coming up in yield while changing over from previous traditional MLC and TLC standards on the flash side + DDR4 taking some capacity from DDR3v lines. All of this has created a substantial demand increase and supplies that are behind that curve.
Supply is likely to to be under pressure with accompanying costs to continue to increase for the next 2-5 months on NAND flash before normalizing and going into a cost per GB decline, ast least that is what seems likely. Memory may stabilize here in 6-12 weeks, but it is questionable if memory prices will come back down soon, looking at demand and capacity to produce.
See also all memory upgrade options.