I'm not sure I'm following you.
(In your calculation, is there supposed to be a "?")
So if I used your calculation for the large bag, would this be correct?
(( = ".75")? + :) * [Quantity] -- my interpretation of your calculation
(([size] = "3/4 of a lb")?[Price] + [lgPrice]:[Price]) * [Quantity] -- your calculation
But if I do all that, why is it not possible to just have two price fields for each product (LgPrice and SmPrice)? Is it because the calculation uses both fields regardless if the person chooses small or large?
Also, how should I handle the different weights?