I have a spreadsheet where I am trying to calculate the profit margin of
individual products. In some cases I have all of the figures I require. In
other cases I am missing values (e.g. price or cost) and generate data is
does not error well.

Can you suggest formulas or method to have missing data error more
eloquently for the last 2 use cases below??

Price | Cost | Margin | % Margin
---------------------------------------
$100 $60 $40 40%
$0 $60 $-60 #DIV/0!
$100 $0 $40 100%

Any help you can offer is greatly appreciated!