If it delivers value to a particular target market, you can in turn deliver the members of that market to advertisers who would be willing to pay for the privilege. If you're going to have a subscription newsletter in today's net world, I think it should deliver a ton of information along with analysis for a very low price. The economics are in your favor. The original Briefing Memo was in print form. It took almost two years to get to breakeven.
Online newsletters can start to pay off right away.
An example of an excellent paid newsletter with lots of information at a low price is Innovation Digest, produced by John Gehl and Susan Douglas. All sorts of folks use free newsletters as business builders. Here are some examples. PCWorld has what may be two of the free newsletters with the largest subscriptions. They both provide tips on technology issues and combined the Dummies Daily and TipWord newsletters send out over two million issues every day.