I'm advertising online for a small business which rents equipment. I'm trying to assign a profit value for each sale that is driven through the advertising and am running into the following challenge. Let's say the business rents furniture and they buy a couch for $100. They rent the couch for $10/Mo. If a sale is made to rent the couch for 6 months the actual profit from the sale is $0. How to I calculate the ROI for the ad?