The estimated average Google search ad cost-per-click (CPC) is $2.69 [0].
Working on the conservative assumptions that an average person will run 10 searches a day and click on 1 search ad a day, Google will make $83.39 a month from the average user. It is likely much more though.
Bear in mind that there is a vast amount of clicking on "wasted" adverts, i.e. paid adverts which would have been the top result of an organic search anyway, e.g. eBay spending $20M a year on ads targeting the keyword "eBay", which they thought was great because it appeared to give a $245.6m return on investment, until they switched it off for a bit and realised they got pretty much the same traffic and conversion rate without the massive advertising spend[0].
Their global ad revenue is a public figure right? It should be easy to estimate an average. If they have a billion users, do they actually have $100B in monthly revenue?
I highly suspect the distribution is not flat in any way though (some users are much much more valuable than others and it's why advertisers pay FB and Google to target the ads for them).
