Bundle Stars has a great deal on both S.T.A.L.K.E.R.: Clear Sky and S.T.A.L.K.E.R.: Call of Pripyat
S.T.A.L.K.E.R.: Clear Sky is currently $1.99 USD a discount of 80%
S.T.A.L.K.E.R.: Call of Pripyat is currently $6.99 USD a discount 65%
And if you wanted to complete your S.T.A.L.K.E.R. collection dont forget you can pick up The Killer Bundle which has S.T.A.L.K.E.R.: Shadow of Chernobyl along with Pixel Piracy, Year Walk (a great game), Alone in the Dark and 6 more for only $4.99 USD
Call of Pripyat is a great game. Wouldn't necessarily recommend Clear Sky though.