What kind of marketing returns could I expect?

[Bob S]

If I spend $15,000 per month on a marketing firm for PR/media relations and buzz/viral services, what kind of returns could I expect? I am looking for a formula, such as that amount of marketing might equal a given number of visitors to the company's website (or something along those lines). I realize that it could vary greatly depending on each individual situation, but there have to be some rules of thumb, right? It would be a U.S.-based campaign, covering radio, tv, internet and print, all based on a website in the social landscape that offers completely free access to all users.