How many people cross your paths on a monthly basis? You can consult Google analytics for this.
How many visitors are interested in buying your product?
What percentage of your leads turn into customers? This is critical to monitor. Talk to sales and make sure the leads you deliver are top notch. Increasing lead to customer rate can drastically improve the ROI.
For many companies this number may vary or increase over time. Test different options, such as a new customer vs. the lifetime value of a customer.