Thanks for the responses! Jessica, that was actually the thread that inspired the question

. I thought of posting there but it seemed like it would be a change of subject so decided to start another one..
Joe, since we work right on the dealership site (and for a few other reasons too), we are able to tell with certainty which are "our leads". Granted, the lead may have originally come from somewhere else but since we provide a unique type of interaction with the customer, once the customer goes through our system, dealers don't have a problem (so far) with letting us claim that lead. This is because its very clear that the lead became an appointment through us (and subsequently/hopefully a sale too).
So since it is possible to tell that we are "responsible" for the sale, do you think its possible/desirable to charge this way? We thought it could be an innovative way to price to value because we only want dealers to pay for us if they can tangibly see what we're bringing..