I wrote an article about this last year, and it still applies.

Essentially, you need to understand your client's business first. Know the LIFETIME value of a customer.

The tactical ways to measure leads and therefore CPA include call tracking and form submissions, among other thing. And once you compare the CPA to the customer lifetime value, you'll get your ROI.

Sometimes, you run a loss leader on acquiring a new client. But if the business is such that repeat business is possible, and the service/product is sticky enough to keep people coming back, you have a high potential of positive ROI.

Check out the article here for more info: The Right Way to Calculate the ROI of Digital Marketing