Ok well you could just do a math formula to calculate it then in the calculations section of the cart object.
Since you don't want to discount the option price, you will have to store the base Price as the price and add a standard column to the cart for the OptionPrice and insert them separately.
DiscountedPrice = [Price] - ([Quantity]>10?[Price]*0.1:0) - ([Quantity]>20?[Price]*0.05:0)
That would give a 10% less than the base price if you order more than ten and 15% if you order more than 20.
Then update your TotalPrice calculation to use that value as well:
TotalPrice = (([Price] - ([Quantity]>10?[Price]*0.1:0) - ([Quantity]>20?[Price]*0.05:0)) * [Quantity]) + ([OptionPrice] * [Quantity])