the monetary cost is misleading. you can't take 4 billion dollars and use it to hire developers for a year and then expect something equivalent to the linux kernel after a year. money doesn't actually translate on its own into valuable things - people have to actually do work to make that possible. and not every task is infinitely divisible such that it can be worked on in parallel by many hands.

the actual cost is in person-years of work, not dollars, and even then you can only speed up the time it takes so much by throwing money at the problem.