You will need to create new columns in the cart to hold the price for each range, then in the add to cart button, have the price for that range for that product added to the cart.
You can then use the price range column in the calculation. So you would have the columns: [Price] [PriceRange1] [PriceRange2] [PriceRange3]
then you could your you calculation as:
(abs([Qunatity]>0 && [Qunatity] <= 5) * ([Price] * [Quantity])) + (abs([Qunatity]>5 && [Qunatity] <= 10) * ([PriceRange1] * [Quantity])) + (abs([Qunatity]>10 && [Qunatity] <= 15) * ([PriceRange2] * [Quantity])) + (abs([Qunatity]>15 ) * ([PriceRange3] * [Quantity]))
Your ranges will be different, I was using 1-5, 6-10, 11-15, greater then 15