Costs for transactions, not events?

Period costs for events seem pretty straightforward, but can you (and should you) assign costs for marketing programs of a transaction, such as a trial download form, to give us an accurate look at costs for ALL channels? The cost per download can be relevant website admin costs divided by number of expected downloads. Under period costs, I see there is a date, and this is clearly a dynamic, non-date cost so period cost doesn't seem to apply here.

Thanks in advance!