Are they really!? In regard to the charge no there isn't one in addition to the initial advertising rate.
I never thought it made THAT much difference- on an average day one of my ads on the forum will get between 60-80 views which I suspect is far less then my main profile ad. However it is an added bonus which I make sure I use to justify advertising on EI which is more expensive than any other place I have ever been to in my life!!