Well, you would need to calculate the marginal cost if you were really interested in what a particular search costs, but I think this number is actually says mu

Well, you would need to calculate the marginal cost if you were really interested in what a particular search costs, but I think this number is actually says much more. 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.