I could explain more of an architecture that would let your price field get populated after the relevant data had been entered, but honestly it would require some additional changes that I don't really think you would find worth it just now.

Instead, consider this: Why not simply display the value of calculated fields in the toString() info that shows in the list entry and drop the text fields for them altogether? There is little need to provide text entry fields for data that the user doesn't need to enter or edit - such as values calculated from other inputs.