Weird that no one has given the true answer:

Where does the 50 BTC come from when mining?

The answer is that it comes by devaluing all the previous Bitcoins by the tiniest amount.

It is exactly the same as the government printing money, it is inflation.  Being that it is predictable and controlled inflation to a set formula, we can assume that that inflation has already been priced in by the existing owners/purchasers of Bitcoins.