The reason for the pricing is a combination of demand and availability. The lower grade shrimp can be bought relatively inexpensively since that fairly common. The higher the grade, the less available the shrimp tend be and the greater the demand.
These shrimp can be fairly sensitive, so you need to have a good handle on the tank parms to keep them alive and get them breeding. I also takes years to breed from a lower grade up to the next grade level, since most of the babies will be the same grade or lower instead of higher grades.