Say I was to publish a small magazine that goes out to between 500 - 1000 people. 50 pages total. How much would you, as an advertiser, be willing to pay for a full page ad in the magazine?
This all depends on the conversion rate of their ads, no one is going to pay for ads if they don't get a return on their investment. Do some trial runs, offer free advertising so they can get their feet wet.