To get a handle on ROI from an online marketing campaign, you need to know what it will cost you and how effective it is/was. Knowing the costs of an online marketing campaign is relatively easy; calculating its effectiveness less so.
Different companies measure effectiveness in different ways. Some say that effectiveness is determined by increases in sales volume (that is, obviously, the most likely response), but others believe that effectiveness is gauged by the number of leads generated by a marketing tactic or program.
It gets messier yet - other metrics abound including number of press releases or pieces of mail, number of new customers, number of web hits or visitors, awareness changes (or the extent to which people are more aware of your product), impressions (number of articles in the press multiplied by the potential readership of each publication), content analysis, recall measures, and on and on and on.
Knowing what you want to achieve with your campaign is the key to measuring effectiveness. Do you want to drive sales volume, web visitors, or what? Once you know that, you will be able to calculate your marketing ROI more accurately.