Rupert Murdoch’s plans to make Wall Street Journal Online free sent the company, readers and Wall Street into a tizzy this week.

Exactly how much more traffic WSJ.com needs to generate to recoup lost subscription revenue is tough to estimate as Dow Jones does not disclose much about the site’s financial performance.

Most analysts estimate Murdoch will need to recover between $50 million to $75 million in annual subscription revenue for WSJ.com.

We sought out the help of media consultant Mike Vorhaus, managing director of Frank N. Magid Associates, who has advised the New York Times, Dow Jones and AOL, for some help.

First of all, let’s assume that WSJ.com’s annual subscription revenue is the midpoint of estimates, or $62.5 million.

Then let’s assume that Dow Jones charges $30 per advertising unit, which is measured in CPM, representing cost per thousand views. (Estimates on how much Dow Jones charges range from $25 to $115, with most premium ads sold around the $30 range.) So that means each ad view is worth 3 cents.

By dividing $62.5 million by 3 cents, you get about 2.1 billion, which is the total number of times the ads have to be seen per year to recoup the subscription revenue.

Next, we need to figure out how many visitors WSJ.com has to attract to get 2.1 billion ad impressions. We looked at comScore data, which indicated viewers from around the globe on average visited WSJ.com about 1.8 times in September, during which they checked out about 12 pages each.

So assuming each page sells two ads at $30 CPMs, and multiplying that by 12 months, Vorhaus figures each user will need to view about 518.4 ads each year.
(1.8 x 12 x 2 x12 = 518.4)

Dividing 2.1 billion ad impressions by 518.4 ads equals 4.018 million unique visitors per month required to recoup the estimated $62.5 million in lost annual subscription revenue.

comScore data showed WSJ.com attracted about 3.1 million unique visitors in September. In other words, WSJ.com needs to boost traffic by about 130 percent.

Left unknown is just how many non-paying visitors currently leave WSJ.com after refusing to pay the annual fee. Some estimates for financial Web sites with annual fees put this number at around 80 percent, Vorhaus said. If so, closing the gap between comScore’s 3.1 million global visitors per month and the estimated goal of 7.1 million unique visitors would not be all that hard.

However, after further reporting, we realized the impact of such a move on WSJ.com in isolation misses the point. Its potential damage to other parts of the business could be far greater than simply recovering subscription fees at WSJ.com.

Meanwhile, check out WSJ.com readers freaking out about a free WSJ.com.