If they sold a package "X ads for X dollars depending on proof of conversion." it might have been something I signed up for. Realistically if they could have done it, they wouldn't have offered any other option.
"Do you want to run ads? This is our package. You pay $X and if you get sales from those ads you pay $X + $Y because we proved the ads made you money. Take it or leave it. We own 85% of the ad market."