The cost was really low because he was mining with his CPU. Is not like nowadays with the crazy mining difficulty where we need tons of PHz to mine.

Any one could fork bitcoin, make a new genesis block and mine the first 1M coins with his CPU, it will take some time:

50btc each 10 minutes means 200000 min. or 138 days to mine 1M coins.

We can visualize with the blockchain data, how satoshi Mine his coins: