This is now, after years of quad core CPU's being available for reasonable prices.
And you want me to believe, that somehow people will jump to 6 or more core cpu's in a matter of 2-3 years? Just forget about physics run solely on CPU already, it's dead. The future is either GPU only or a combination of CPU and a GPU, still with the GPU doing most of the work. Which PhysX already supports btw.
By the time everyone has 6 or more core CPU's, the min. for GPU physics is gonna be something much more faster than a 9600GT and you'll be right back at the beginning.
So no, the comparison is not just the cost, but it's cost, two sockets not being standard, people that upgrade GPU's much more often than CPU's and so on.