In other words, advertising does not have to be the sole source of revenue and you should definitely take these other things into consideration. If you're not using these sources of revenue, that doesn't mean that they couldn't be used. You can always create more revenue out of existing eyeballs. A single subscriber could refer 10 or 20 of her friends to your ezine. Saying that this subscriber is only worth the share of the advertising space that she creates misses this point completely.
Loyal subscribers tend to create more subscribers, thus increasing their value.
I mentioned yesterday that if you could somehow track which subscribers buy from you or from your advertisers, their value would increase greatly. While software exists that will do this, it's unfortunately very expensive at this time, and the costs far outweigh the benefits for most folks. As time goes on however, this technology should become available to everyone at a much cheaper price. Assuming you're providing them with information they value, they begin to regard you as an expert in your field. That's a lot of power, and an element of an ezine's worth on which I'd have a hard time putting a monetary value.