I have a 200 50 ZR 17 m/c 75w Metzler Radial tire, the price is $285 with free mounting and balance, but a 200 55 17 radial is $225, what gives, why is the 50 so much higher and what is the difference between the 2, both are Metzlers by the way.

 

A 'Z' tyre is rated for very high speeds, over 150mph, so has to be manufactured to suitable high standards. You don't say what speed rating the 55 profile is, but I would guess it has a lower speed rating, such as H.