>If we're really digging into it, the actual value of a company should be the amount of money you need in total to buy it. Including the parts that are not for sale (so not just the public shares)

That doesn't make any sense. The "actual value" of something must include what people are willing to pay for it, not what the existing owners are willing to sell at. If I have a trinket that I'm not willing to let go for less than $1M, the "actual value" isn't suddenly $1M.