GamersGate has S.T.A.L.K.E.R.: Clear Sky (Windows PC) for $2.49 downloaded. Clear Sky is not so much a sequel to Shadow of Chernobyl as it is a alternate look at the original game.
GoGamer has S.T.A.L.K.E.R.: Clear Sky for the PC priced at $6 + $3 shipping = $9 shipped. Clear Sky is not so much a sequel to Shadow of Chernobyl as it is a alternate look at the original game.