As I understand the question, you're asking whether it's a once-off cost to buy the special forces add-on?
Once you have the game, patches are free. As I said here the last time you asked that, 3 hours ago. There is other downloadable content that you can get if you pay for it, but it isn't necessary to run or to enjoy the game. You only need to buy them once. Steam knows you have them, and you can re-download them through Steam (without extra cost) if you ever uninstall and re-install the game elsewhere.
Oh, and wrong forum. Please, for the love of God, read the forum descriptions before making threads.
"Into the face of the young man who sat on the terrace of the Hotel Magnifique at Cannes there had crept a look of furtive shame, the shifty, hangdog look which announces that an Englishman is about to talk French." - P.G. Wodehouse, The Luck of the Bodkins