Snapshot calculations

I have a master price table containing part numbers and customer prices and a child table that adds parts to a customer work order table. I set up a snapshot field to in the child table to capture the current price of a part being added from the master price table. 

When the part number is selected from the master price table, and  before save is clicked, I want to do some calculations on the current price such calculating Qty. x current price and the sales tax for the part. 

The problem is the snapshot field isn’t valued until after the record is saved so my calculations for the extended price (Qty x Price) and sales tax don’t display any data until after the record is saved.  

Is there a way to do calculations on the snapshot field before saving?

I don’t want to base calculations on the lookup field in the master table because I only want the calculations to be based upon the part price when it was first saved