I don't see why we should pay website owners through Google, who will presumably take a cut of the money, when decentralized digital currency exists. I'd like to see a Bitcoin based system in the form of a browser plugin.
Do you want people to click to donate every time they show up on a page? People will never click that button.
This system is great because it's a) seamless (you just get one bill a month), b) based on usage (you visited these pages) and c) is a market (the price you pay is based on competitive rates that advertisers would otherwise be willing to pay).
You can cover all of the points you made by loading a Bitcoin wallet and having the plugin deduct fractions of a BTC when you visit these web pages, and when the wallet is empty ads are loaded instead. The main loser in this situation is Google, since the site creators get all of the money. Users may have privacy concerns since BTC transactions are on a public record, but you can keep creating new Bitcoin wallets automatically to mitigate this concern.
A user's client should respond with an accept/reject response to the price offered by the website. Everything else would be determined freely: Clients can be created with price range configurations as desired (perhaps per website, in a probabilistic way, and/or some cost-concious settings), and website owners can analyze their visitor's accept/reject rates to determine how much to ask for before falling back on ads.
This system is great because it's a) seamless (you just get one bill a month), b) based on usage (you visited these pages) and c) is a market (the price you pay is based on competitive rates that advertisers would otherwise be willing to pay).