I'm trying a different approach (don't know if it will work yet :-). I am trying to charge different for different categories, based on the population of the category.
I think what the market is willing to pay varies alot on what the category it is in. For instance, a law firm will likely pay more than a plumber.
I guess that might explain the rise in bidding directories.
I also think that a large part of Yahoo's demise was because they had one price for all. It completely blocked out large segments of the market and Pay Per Click was able to step into the void left.
The big problem I am having is getting the link population up to where the idea will flesh itself out. I offer free links as well, in the lower populated categories and I then have to move the category through a transition. Might be too much hassle. but don't know yet.