I dont know about prices but is atmel with 12 clock mode cheaper than philips with 6 clock mode?
Most "plain vanilla" devices are 12 clockers, the less than 12 clockers tend to have fancy stuff such as PCA, Timer2, ERAM, Large Flash and such. This is absolywunnerful if you need it, if not you will tend to use a simple 12 clocker.

I do not think much of the price differential is due to the clock multiplication.

Erik