This is my calculation which I use when I want to see if there is a business case behind a media investment for my clients or my own marketing initiatives.
There are so much information available today, and from that you can manually calculate predictions and potentials before you choose to invest in a specific media, channel or campaign. Armed with the right specific, relevant numbers for your business and some industry averages you can create a hypothesis for the potential for an investment if you are unsure, or just want to make a calculation in terms how you should re-distribute your media budgets and what the potential could be.
Marketing today, merges business savviness and analytical skills. There is just a question of time (I guess there already is someone out there who has turned this into a formula, if not, that someone could make some nice bit of cash on a software for this, collecting data from advertisers, merged with some kind of ad score to calculate predictions based on data available).