For those not familiar with Impulse it's a digital distribution platform like Steam but is PC only.
Next cheapest is £14.95 @ Zavvi
With over 2 million copies sold, the new episode of the most internationally successful S.T.A.L.K.E.R. series seamlessly connects to the first part of the Shadow of Chernobyl. S.T.A.L.K.E.R.: Call of Pripyat takes PC gamers once again into the vicinity of the Chernobyl nuclear reactor that exploded in 1986. This so-called "Zone" is a highly contaminated area cordoned off by the military and now is combed through by the so-called stalkers, modern fortune hunters, in search of unique artifacts. In the role of Agent Alexander Degtyarev, who is dressed up as stalker, the player has to investigate the mysterious disappearance of five military helicopters and thereby stand ones ground against the various stalker groups