You can get S.T.A.L.K.E.R.: Clear Sky for as little as £0.05 via a key-selling website called Gamivo.
This deal is for the GOG version of the game.
Scroll down to the “Offers” header on the page.
Choose the cheapest offer and click “buy now”.
Go to checkout.
Untick the “Customer Protection program” checkbox.
Confirm you do not want this by clicking the conveniently hard-to-see grey link in the pop-up.
Add the following discount code:
Once purchased, you can redeem your ultra cheap GOG key here: https://www.gog.com/redeem.
Note: Sometimes if the price in the Offers section is low enough, you may even be able to get the game for free with the discount code!
Tip: Check that you do not already own the game on GOG before spending your pennies.