For a fairly small site/forum, with about 1,000 unique views a month, 600 members (though not all of those active of course) and of course the banner would be targetted as being things they might like. The banners would be randomised rather than static, and would rotate through perhaps 20?
How much would the banner ad be worth for a month, in your opinion? Especially for things such as webcomics or blogs, because I can guarantee that the members would be interested in those, rather than for things for sale because of lack of disposeable income amongst students.