M PSS Tires...Why are rears so expensive compared to other brands and the fronts are cheap?

I have a G37 Sedan S w/18" wheels. Front 225/50 and rear 245/45. I might upsize to the 255 in the rear. Someone told me to get the 255/40 in the rear. Now sure why the downsize from 45 to 40...maybe he told me wrong.

The PSS rear tires cost more than rear oem BSs and the front PSS are $100 less. Why?

 

I'm not sure if you understand tire sizing but when you look at tire sizes, 245/45 e.g., the second number indicates that the sidewall height is 45% of the 245 mm width. So if you upsize to a 255 you want to reduce your sidewall height to 40% of the larger 255 mm width.
This is so your total circumference remains the same so your speedometer isn't thrown off.

As far as expense; I'm not sure about the large discrepancy but usually you gotta pay to play.