I would have laughed semi-snidely at this six months ago, but these days things are different. CWC Software, the company behind QuickFill which is one of the best-loved circulation management software packages for small-medium paid subscription print publishers, is offering a free Breakeven calculator download from its site (fill out the form at the link below and the next page will give you the free download plus a handy four-page PDF report on the art and science of calculating return from subscription
The thing is, this calculator assumes that you are marketing 100% through direct postal mail. Which is so costly compared to Internet marketing as to have been a laughable option for any subscription marketer who knew better; until recently. What's changed? Well, the spam-surge means email marketing is getting harder, just as the current dearth of postal mail campaigns means you can actually get much higher than expected response rates offline these days. I'm not saying DM will ever replace email (horrors!), but it might be a good supplement, especially for folks selling e-subscriptions in a tight niche market.
You might also be able to adapt the calculator pretty easily to use it for paid search engine marketing.
In the meantime, I'm personally frustrated by the fact that companies like CWC, who've spent years understanding the complex needs of the business of paid circulation (and it's more complex than you may wanna know), are still crawling when it comes to serving online publishers. In fact, when you check out most traditional circ tech companies' Web sites to learn about their "Internet" services, generally what they mean is you can access your customer records via the Internet, not that you can sell subscriptions to Web sites.
My fantasy is that one day the smartest of the old dogs will merge with the smartest of the new dogs -- tech co's that specialize in all the now-classic online stuff like subscriber DRM, instant credit card processing, tracking affiliate sales, month-to-month charges, etc. -- and we'll get one company with the best of both worlds to help us. Dream a little dream.