OK, you probably want to add calculations like this:
TruePrice = ((abs([Size]="2XL")*[XLPrice])+(abs([Size]<>"2XL")*[Price]))
TotalPrice = ((abs([Size]="2XL")*[XLPrice])+(abs([Size]<>"2XL")*[Price])) * [Quantity]
Then on your page show the value of TruePrice instead of Price and it should work. Is the column total correct now?