I haven't reied them out, but i doubt very much that they get a lot of traffic. As you mentioned i doubt they get seen a lot and the top banners are pretty much in your face the whole time.

As with everything only testing it will give you an answer i'm afraid, as all banners will convert differently too.

I never even actually noticed not until you asked lol

If the bottom banner ad wasn't located beneath the posting rules under the quick reply box, then they would be more visible to the page visitor. Now, they are barely noticeable. Though it seems someone is advertising there, so it must be worth something to them.

Based on the comments in this thread alone, my guess is not very good.

I would never buy them. They are too far down in no mans land and no one has any reason to be scrolling down that low. They definitely are not worth half the price of the top ones.