Calculate contribution before ROI
Multiply incremental revenue by the contribution margin before PPC cost. Subtract the full channel investment, then divide that difference by the investment and multiply by 100. At $15,000 of incremental revenue, a 40% contribution margin, and $3,000 of investment, contribution is $6,000; net contribution is $3,000; scenario ROI is 100%.
Keep costs and revenue consistent
The margin should include relevant product, fulfillment, and payment costs but exclude the same PPC investment entered separately. The investment should include media or agency fees, internal labor, tools, and production that belong to the analysis. Use a defined attribution window and do not mix lifetime revenue with one month's campaign cost unless that is an explicitly modeled cohort comparison.
Attribution is the hard part
A dashboard may report revenue associated with a click or an organic-search visit, but that is not necessarily revenue caused by the channel. Returning customers, other campaigns, seasonality, and brand demand can all affect the total. Compare with a baseline or control where feasible, show a range when uncertain, and label modeled revenue separately from observed sales.