Prices will vary by region and states.
MA, NY, and CA have legal restrictions that drive up the price of the available guns. Some states, like Texas, have much lower prices and higher demand.
I have the Model 649 and it's quickly become a favorite gun. I like the unusual looks of the 649 slightly more than the 640.
For what it's worth, I found that the 649 seemed to be regulated for 158gr ammo in either .38 or .357. Downside is that the Magnum loads are a tad more accurate, but recoil of 158gr .357's are stout enough that I have to readjust my grip after each shot. The good news is that 125gr JHPs seem to be nearly as accurate and much more pleasant to shoot.