Hi,
just create three TextFields (Cost,Margin and "Selling Price") and switch them to "Number"-Format.
Then set the "default" property of "Selling Price" to "Cost.Text+Margin.Text"
If you now update
1) "Selling Price" : then you get that number
2) "Cost" or "Margin" : then "Selling Price" contains the sum of "Cost" and "Margin"
Best Regards,
Sven
Best Regards