A more useful analysis would be to estimate the marginal cost of each additional search (e.g. if I go make a search right now, how much extra does that cost Google?).
Likewise, there is a similar number that represents the marginal cost of each additional webpage that Google indexes.
I'm not sure if there has been any innovation in this area (shut off power AND cooling for certain areas in the data center)
To cut down on cooling, you need to architect your farm to have smaller, independent cooling systems. However, your system must also be architected such that weird things like "only machine is active in my cooling cluster, so I cant shut down the cooling for that area" cannot happen.
I'm not sure if things like that have been solved.
It is the amount that Google is spending on average on a search and thus it is the amount that Google has to earn back on average per search in order to be profitable.
Interesting, but tricky. Is the price of the new servers they need to buy for the peak processing marginal or fixed cost?
It interested me to know that, based on the average MPG of cars in the US  and a rough cost of a gallon of petrol , it would cost 1.6% of a GBP penny, or 1/40th of a USD cent, to emit the same amount of CO2 in a car as a single Google search (0.2 grams of CO2 ).
I'm not sure why I felt the above was interesting enough to write in a comment, but for some reason it does interest me that, if Bill's calculations are correct, it costs Google 27 times as much per gram of CO2 emitted as it does drivers (excluding car costs etc.) Even assuming I'm showing signs of insanity chosing to work that out, the amount of CO2 emitted is good to know.
That's a very significant piece of the emissions footprint of a car. I've seen numbers that say as much as 30% of the emissions resulting from a car come from the car's construction and distribution.
Anyway, I found his transparency and links to the sources way more interesting than the actual result (which I somehow still doubt on an intuitive level)
I would think (but cannot confirm) that Google makes more per search than Bing or Yahoo.
A user is a user is a user. Unless google users are that much more likely to buy something after clicking my ad, why would you pay more per individual view? Sure the absolute numbers should go up with more views, but it doesn't seem like money well spent to pay more for similar adspace.