DDR2 memory prices have been recently going up thanks to supply and demand catching up with the computer memory industry. Sets of DDR2 RAM that cost $40 just a month ago are now nearing $100. At the same time, DDR3 prices are falling slowly, as more computer enthusiasts use the new technology in desktop builds, and computer makers start to offer DDR3 memory in lower priced systems.
What this means for the computer memory industry is that DDR3 sales will be taking over DDR2 sales, and technology will be moving forward once again. DDR3 prices are now affordable and are still dropping. Due to this, much less people are buying DDR2, so the price is rising. DDR3 is about to become cheaper than DDR2 due to this effect.
If you need DDR2 memory it is best to buy now, before prices rise even more. As the technology gets older, demand will fall, and prices will rise. If you plan to keep DDR2 compatible motherboards for a while, stock up on the RAM. If you end up not needing it later, you will most likely be able to sell it at the same price you bought it for, or even higher, and make a profit. Now we wait for DDR4.