A revenue:cost ratio of 5:1 is generally thought to be acceptable for marketing spend, including law firm internet marketing. In other words, your average cost per acquisition (CPA), or the cost to acquire each case, should be about ⅕ of the average value of each case. This includes pay-per-click costs, web design costs, your own time, and anything else,
Your cost per acquisition (CPA) may be high since law firm online marketing can be so competitive, but if your case value is high, it may still be a good ROI. As an extreme example, some of the most expensive search engine marketing terms are those around mesothelioma lawsuits, with firms paying hundreds of dollars per ad click (for reference, costs are often a few dollars or cents). This investment provides a good ROI, however, as just one settlement will average over $1 million and cases are often handled by personal injury lawyers who bill on a contingency basis.
The important thing is to have a marketing tool that helps you track your CPA accurately, and a practice management software where you can track the average value of a case, so you can always have insight into your ROI.