It is 100% free, I got mine from here. The giveaway is sponsored by AMAZON so don't be afraid of taking advantage of it. Note that you are required to have a card (debit/credit dosen't matter you won't pay a thing to amazon)Here are the steps.
1) Go here:
2) Fill out name and email (you can use fake name and email)
3) Get Amazon code for Thief Gold.
4) Go here:
5) Click Purchase
6) Apply Amazon code from Step 3 to bring price to $0.00.
7) Buy game for $0.00.
8)
TO GET STEAM KEY Go to YOUR ACCOUNT>YOUR GAMES&SOFTWARE LIBRARY. At Thief Gold you will find down there the Product Code. That is the code you have to redeem on steam