I have a form where you can select which parts were purchased for a service order. The parts are selected from a 'parts' form which includes fields for 'Part Name' and 'Part Cost.'

The user can select multiple parts in the multiselect picklist.

I would like to display the total parts cost based on the selected parts. How would I go about doing this?

As an example, in form PartsList I may have

Part_Name      Part_Cost

Part1            $10

Part2            $20

Part3            $30

If in field  Service_Report.Parts_Purchased, the user selects Part1 and Part3, how can I update the field Service_Report.Parts_Total_Cost to the total ($10+$30 = $40)?

Thanks in advance for your advice!
Cheers,
Elizabeth