Really? Because I think it's very much the same thing. I think that a company which wishes to invest should always compare to the price of a green-field invest

Really? Because I think it's very much the same thing. I think that a company which wishes to invest should always compare to the price of a green-field investment. If someone were purchasing Uber for $18 billion, they should do so only if it would be more effective then dumping 18 billion into a green-field startup. If someone is investing by purchasing some shares at an $18 billion valuation then the comparison is slightly harder: own 100% of a startup with a much lower investment or x% of the market leader. The comparison is now always trivial, but I think that anyone looking at these sorts of investments needs to be doing this comparison before spending the money.