S.T.A.L.K.E.R.: Call of Pripyat £14.99 if you already own the Steam Version of S.T.A.L.K.E.R. Clear Sky or S.T.A.L.K.E.R. Shadow of Chernobyl. Please note that you need to have Clear Sky or Shadow of Chernobyl registered to your Steam account before purchasing Call of Pripyat to receive the discounted price.
Or £19.99 if you don't already own the Steam versions of either game.