Have you checked the price of 168-pin non-DDR SDRAM lately? Newegg doesn't sell it, so you'll have to check somewhere else. Here in Mexico, this memory costs twice as much as DDR memory (DDR2 is still relatively new; DDR is still the most common type).
It's a simple supply & demand thing. Manufacturers move towards the newer standard, so supply of the old stuff drops. However, the market takes far longer to adopt the new standard, so demand doesn't change much. Result: price increases.