you would need a column for minimum and maximum quantity for each price.
for example:
priceA : 10
minA: 1
maxA: 10
priceB: 9
minB: 11
maxB: 20
then use a calculation for the price of:
(abs([Quantity] <= [minA] && [Quantity] >= [maxA])?[priceA]:0) + (abs([Quantity] <= [minB] && [Quantity] >= [maxB])?[priceB]:0)