I guess more than quantity, prices are based on quality. For instance, top notch people read Link Exchange Digest, and even their subscription is substantial. So it must all depend on the type of readership. It depends on us to which ezine we submit our ad and what price we are ready to bear for what type of readership. For that we must go through lots of them, observe what material is being churned out, how responsive are the subscribers, and how good are the editors and moderators.
Email advertising is a powerful tool, but only if the positioning is right.
Again it's whatever you can sell it for. I set up a webpage mentioning the amount an ad costs, and link it to the payment processing site. I don't think my processing service would accept this system. With online checks, the process is more complex. The Email List-Advertising Resource Center.