I was given a figure of 24k to advertise for a year at CF. I don't know that for sure, just a word of mouth thing.
I was once informed by an admin over there that whatever the price, it was worth it for the opportunity to reach that many people at one time...wow!
Of course this was in the steering committee room (supporting members and such only) while discussing the "merits" of pop up ads that directly completed with the paid vendors sales