Launched June 2003, Waterfront Media's paid subscription Web site for Rodale's bestselling book South Beach Diet, has already pulled in more than an estimated $20 million in revenues and counting.

Although paid subscription sites have been around for a decade, and red-hot for half that time, Waterfront Media's SouthBeachDiet.com success made us wonder if there's a whole new window of opportunity that could blow the lid off online sub sites.

Two former Rodale execs, Joe Berkowitz and Caren Sinclair-Kay, co-founders Red Jam Media, are among the new wave of entrepreneurs dedicated to turning offline famous-name content brands into subscription sites.

We asked them, "What sorts of books, magazines, videos ... offline content can be turned into paid subscription sites, and how can you predict success?"

Here are their "Can it fly?" paid sub launch criteria to inspire you.

Criteria #1. Substitution value

Can your site visitors find pretty darn similar content for free elsewhere on the Web? The brand name you're launching a sub site for better be extraordinarily strong to persuade even the casual eye that this site is UNIQUELY worth paying for.

Example -- despite the "ton" of free info on real estate online, Red Jam Media is fairly sure that if Donald Trump launched a subscription site based on his new real estate book, it would be fairly successful.

Criteria #2. Is it already selling in multiple media?

Has the brand already expanded across multiple paid offline media -- such as books, videos, audio tapes, live events, etc.? The likelihood consumers will ante up for a sub site is very high. For example, Deepak Chopra or Suze Orman could probably do well with subscription sites.

Criteria #3. Can it spawn interactive tools?

Unlike books or videos a consumer buys once, a subscription is an ongoing sale. So you need content that will continually engage and interest them. What interests people the most? Themselves.

So, in addition to frequently freshened site content, a hardy set of interactive tools is critical to keeping those autorenews continually renewing. Examples:

-- A personal progress chart and/or diary -- Personal planner and emailed reminders for personal goals -- Personalized tip sheets (printable or wireless) -- Interaction (chats, message boards) with the guru and the community of fellow fans

Criteria #4. Potential ad revenue losses (or gains)

If you're taking online content from free to fee, then obviously you'll be doing the math trying to see if subscriptions will cover the loss of a large portion of eyeballs to sell ads against. In some markets, for example scrapbooking, there are very few large national advertisers spending online as it is, so you're not missing much.

On the other hand, don't assume your sub site has to be entirely ad-free. In some markets (think Hollywood Reporter) ads add value and readers would miss them. Plus, while you may need to pull down overly-intrusive ads from a gated site, you might be able to make up the extra revenue by getting a higher CPM because visitors passionate enough to pay are inherently more valuable to some sponsors.

Criteria #5. Is the market size worth the effort?

Example: While there may be millions of scrapbookers in the US, only a certain percent are online and of these only a certain percent are going to ever visit your site, and of these only a certain percent will pay.

Once you boil it down, that big number may be too thinly sliced to be worth going after.

Criteria #6. Did your online survey pan out?

The Red Jam Media team always posts a survey on a related free-access site to gauge overall market interest. "You're always going to be surprised at results," notes Sinclair-Kay. "We had one concept we thought was slam dunk fail."

Key: Don't simply ask "How much would you pay for a subscription site?" You'll only get reliable results from price testing, which you're not ready to do yet anyway.

Instead, describe the features (especially the personalized tools) and ask, "How interested would you be in getting access to this?" The answers don't include a specific price, but rather range from "very," "somewhat," "only if it's free" and "not."

Criteria #7. Is the content "life event" related?

Life event marketing is incredibly powerful but limited in terms of account lifetime value (consider TheKnot.com). A life event could be anything from a divorce, putting parents into eldercare, buying a house, etc.

As it is, the average B-to-C subscription site charging month-to-month we know of gets about four-seven months of revenues before an individual account goes away. If your bottom line requires more than that and you're not planning any ancillary revenue streams, you should definitely stay away from life events or consider pay-per-view offerings.

On the other hand, if you're marketing to women, life events can be more profitable because they tend to advise other women to do whatever they found helpful. One happy former female subscriber can yield a wealth of referred new accounts.

Useful links related to this article

Red Jam Media: http://www.redjammedia.com

SouthBeachDiet.com: http://www.southbeachdiet.com

Waterfront Media: http://www.waterfrontmedia.com