It seems we are in the middle of a slight memory shortage. Both DDR2 and DDR3 have been steadily increasing in price in recent months. Here in Vancouver, it was not unheard of to see 4GBs of DDR2 for around $40 CDN on sale, last summer -- now, prices have doubled, to around the $90 CDN mark for an inexpensive 2x2GB kit.
And if you think you might be able to save some cash by sticking with DDR2 instead of DDR3 with a new computer purchase, think again. Across North America, DDR2 and DDR3 have reached a rough price paritity.
According to EETimes quoting Samsung, one of the world's largest memory makers, the memory market is in a "sensitive point" and shortages of "DDR2 are more severe than DDR3." DDR2 has been in over-abudance for the last few years, and most production has since shifted to DDR3. A small boom in demand for DDR3 has lowered overall supply, and the large stockpiles of DDR2 are finally drying up a bit.
As we go forward from this point, it seems likely that DDR3 will remain cheaper than DDR2.