Google had a service, now discontinued, that allowed a user to enter into those bids, setting a monthly limit on ad spend. It was for external sites using AdSense, not Google Search, but I thought it was an amazing idea.

> if the user wins the auction, [a thank you message] is placed in the ad space, and the cost of the ad is deducted from the user's monthly contribution. If the user does not win, the winning ad is displayed as normal and the user pays nothing for that slot

https://en.wikipedia.org/wiki/Google_Contributor