You want to generate leads with your AdWords campaign by encouraging people to fill out an interest form on your website. What do you need to know to measure return on investment (ROI) for this campaign?
How much you’ve spent on the campaign compared to the value of leads generated
You can’t calculate return on investment for campaigns that are focused on online leads
The percentage of budget spent compared to how many forms were completed
The number of clicks your ad received divided by the number of times it showed
Whether you use AdWords to increase sales, generate leads or drive other valuable customer activity, it’s a good idea to measure your return on investment (ROI). Knowing your ROI helps you evaluate whether the money you’re spending on AdWords advertising is going to a good cause: healthy profits for your business.
In general, ROI is the most important measurement for advertisers because it shows the real effect that PPC campaign has on your business. It shows how your ads are contributing to the success of your business. ROI is the ratio of your net profit to your costs.
To calculate ROI, take the revenue that resulted from your ads, subtract your overall costs, then divide by your overall costs: ROI = (Revenue – Cost of goods sold) / Cost of goods sold.
In order to calculate ROI for leads, you need to set a value. Filling a form is also a conversion which should have a value set. If your business generates leads, the cost of goods sold is just your advertising costs, and your revenue is the amount you make on a typical lead.
How much profit you’ve made from your ads compared to how much you’ve spent on those ads. Return on investment (known as ROI) measures the ratio of your profits to your advertising costs.
Read more here: https://support.google.com/adwords/answer/14090?hl=en