Dishonoured is a very good game from what I've heard and I'm considering picking it up for myself.
However you should get it off www.greenmangaming.com rather than Steam (You still get a code to activate on Steam so it's the same product) where it is 50% and you can apply a voucher code for 25% extra (Note this is 25% off of the discounted price, not 75% off).
The code is GMG25-G4VDR-0ZL4Q
By doing this you can pick the game up for £11.25 which is an absolute steal for a game this recent and so well received.