How can I forecast advertising sales for a new ezine?

I am working on an ezine project that will allow distributors direct marketing access to resellers in a social setting. I need to forecast advertising sales in order to judge the project's profitability. I am looking for the advertising rates for industry specific websites or how much a distributor might pay to advertise on such a site.

Distributors will not pay advertising fees – retailers and brands will. Distributors are the middle guys with no margin.