Henning's answer is clever, but is not an answer I would expect a (strategy consulting) interviewer to be happy with (sorry Henning!). They want you to do a market sizing here.
As such, the clear clarifying question here would be: "Is it safe to assume this is the price/value of earth to humans?" To which, the answer would surely be yes.
Then, you need to state that price = WTP = value. Value = everything that the earth can provide.
So, you'll want to split up the earth by its major producing parts. This would be Land (surface), Water, and Materials (below surface land).
Estimate the value of agriculture (trees, crops, etc.), fishing, and materials (common earth metals like iron and copper as well as rare earth metals such as Lithium. Don't forget oil, gas, and coal!).
Estimate the world's GDP (you should know the US's, know that China is close to the same, then use the law of 80/20 to calculate the rest). Look at global growth rates. Use NPV calculation to determine NPV of Economic production of the earth.