Typical line item cost calculations look like this:
Qty * Unit Price
Where Unit Price stores a value auto-entered from a related table. If you have two unit prices copied into two fields of your LineItems, this could be the calculation that you need:
If ( Not Isempty ( checkboxField ) ; DiscountPrice ; RegularPrice ) * Qty
Thanks Phil, I'll give that a try. So that calc will go under auto enter section.
It could be set up as either an auto-entered calculation or in a field of type calculation.