Many ezine publishers start charging for advertising when they reach 1000 subscribers. Classified ads are generally the cheapest. They tend to be the shortest ads, scattered throughout the newsletter. Subscribers may or may not see them depending on which parts of the ezine they read. Sponsor ads are longer and placed at the top of the ezine for best exposure.
Readers are much more likely to see it, and therefore response rates are better.
There is usually only one sponsor ad for each issue of the ezine. Exclusive mailings or solo ads are the most expensive - and it's justified, because they provide the best exposure. They are entirely separate mailings from the main newsletter, and the only thing in the mailing is the advertiser's message. The more ads you have in a newsletter, the less effective they'll be. Naturally you can bring in more advertising dollars if you offer more ads.