The AMD dual-core processors cost twice as much or even more than their single-core counterparts. So they arent exactly putting any "pressure" on the single-core parts for their prices to reduce.
Eg: An X2 3800+ (2 x 2Ghz dual core/ 2 x 512KB L2 cache) costs around ~ 16K. A single core 3200+ (2Ghz/512KB L2 cache) costs around ~ 7K.
Of course in the domain of higher-end single core processors like the 3700+ or 4000+, it makes more economic sense to go for a dual-core processor instead.