Depending on the publication, the readers may range from almost psychopathically loyal to the publication to only casually invested, so it really pays to understand the publication and the kind of relationship it has with its readers.
You have many choices of ad types to consider, including text ads, banner ads and solo ads (where the publisher mails your solo ad alone to its list, in effect giving your ad an implied endorsement which can be very powerful).
Ezine publishers range from tiny one person operations with 500 subscribers to large newsletter publishers with millions of subscribers spread across multiple newsletter categories.
An example of one such large ezine publisher I have advertised with with good results is Arcamax (www.Arcamax.com). Currently their subscriber list exceeds 2.2 million across many different newsletters. As of this writing their minimum order is $500.
The Directory of EzinesThe premier online resource for learning about ezine advertising is Charlie Page "s Directory of Ezines. This is a paid service that lists the best quality ezines that accept paid advertising, in many different niche categories.
The DOE is a huge time saver, since its focus is on quality ezines which accept advertising. Yes, you could search for these yourself but you"ll quickly discover that many ezines do not accept advertising, and many that do are poor quality and not worth your time and money.
There are so many thousands of them in fact that it would probably take you weeks or months of research to separate the wheat from the chaff in your particular market. So why bother when the DOE delivers it to you ready to go? This is a perfect example of outsourcing something that would take you months of effort to figure out on your own.