Since Brin and Page have total net worth about $40 billion, the remarks above suggest that they produced between 15x and 225x as much value as their aggregate earnings.

It would be more accurate to say that value of 15X to 225X their aggregate earnings was created via a path that happened to include Brin and Page at a key point. It would be an error to, for example, suggest that this means if we went back in time and... used some crude intervention to prevent Brin and Page from contributing then we could expect a result of an earth in 2013 that contains 9 trillion less in value.

One can estimate the value of creating Google by trying to place oneself in the shoes of somebody with average US earnings in 2000 (~$45,000/yr) and thinking about how much one would be willing to pay for Google to have been created, rather than its counterfactual replacement some years later.

I suspect that this estimate of value creation while interesting is rather likely to be used with misleading connotations. For example using it in a naive manner would result in 'altruistic' decision making that is grossly miscalibrated with respect to actually increasing expected value.