Portal 2, PS3/PC/Mac – £21.25
Portal 2, PS3/PC/Mac – £21.25 delivered
Apply coupon “15MAY-1″.
After having registered the free PC/Mac version, you can trade in the PS3 version at Gamestation for £29 via CEX pricematch.
Edit: Or take it to CEX and get cash. You’ll get Portal 2 for effectively free, and get £4.75 profit for your trouble.