Return on investment
This figure can be found by a calculation of the Revenue Generated subtracted by the Cost. Then divided by the cost and multiplied by 100. For example:
If your advertising costs for the past week were £500 and you’ve sold £1,000 worth of inventory as a result, you have a 100% ROI for the week ((£1000-£500) divided by £500). To express ROI as a percentage, you multiply the result of this formula by 100.
Wikipeida has a good entry for Return on Investment and it can be found here
I have a link to a video that explains ROI related to Pay Per Click on Google Adwords.
