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.
The 50BTC comes from the miner being permitted by the network to add a transaction of 50BTC with an address of a public key that the miner controls to the newly generated block. You are correct that this devalues btc a little tiny bit, but the system has inbuilt deflation by reducing the amount of BTC you get for solving a block (currently 50, will reduce to 25 next year) so, over time, the whole currency will deflate rather than inflate.
Will