My firm often structures our deals this way and we love showing ROI to our customers. The best and easiest way to do this is to ask your client what their average revenue is per sale and average profit margin, and then calculate ROI from there.

It's too complicated to get into the books and show exact ROI, but most customers are just fine using estimated internal costs to prove ROI. We don't pull the trigger on a deal unless we feel we can provide 500% ROI which provides plenty of wiggle room on either side of the deal for unexpected events.