What's the gross click rate? If I run a site that has ads on it, almost all visitors have to download and execute kilo- or megabytes of javascript. Very few users click on an ad.

When an ad impression leads to a sale of something, that's great for that user and the vendor. But how many megabytes of javascript was downloaded and executed by all the users that saw the ad and didn't buy anything? The burden on the rest of us to support those two parties seems excessive.