you will need to create a calculation for this.
on the calculation tab create a new calculation as:
realPrice:
php:
((abs([Quantity] >= 1 && [Quantity] <= 4):[PriceA]:0) + (abs([Quantity] >= 5 && [Quantity] <= 8):[PriceB]:0))
you can keep building this up with the various quantity breaks. you will also need to edit the Total Price calculation to:
php:
((abs([Quantity] >= 1 && [Quantity] <= 4):[PriceA]:0) + (abs([Quantity] >= 5 && [Quantity] <= 8):[PriceB]:0)) * [Quantity]
In the cart display, you will want to change the Price to use the realPrice calculation, and also in the code that passes the order information to the gateway, set the item price to use the realPrice.
