This is the complete Counter Strike pack and it includes CS-GO. If you do not have the games already, this is a pretty good price to get into it.
Not sure if it has been cheaper before, but even if it has, right now this seems to be the cheapest price for the complete pack. Steam itself has it for $29.22 US.
NOTE: I think Greenmangaming has abolished referral links so no use putting them up.
You will get the following games:
Counter-Strike
Counter-Strike: Condition Zero
Counter-Strike: Source
Counter-Strike: Global Offensive
From the Steam page:
"Counter-Strike
Play the world's number 1 online action game. Engage in an incredibly realistic brand of terrorist warfare in this wildly popular team-based game. Ally with teammates to complete strategic missions. Take out enemy sites. Rescue hostages. Your role affects your team's success. Your team's success affects your role.
Counter-Strike: Condition Zero
With its extensive Tour of Duty campaign, a near-limitless number of skirmish modes, updates and new content for Counter-Strike's award-winning multiplayer game play, plus over 12 bonus single player missions, Counter-Strike: Condition Zero is a tremendous offering of single and multiplayer content.
Counter-Strike: Source
THE NEXT INSTALLMENT OF THE WORLD'S # 1 ONLINE ACTION GAME Counter-Strike: Source blends Counter-Strike's award-winning teamplay action with the advanced technology of Sourceā¢ technology. Featuring state of the art graphics, all new sounds, and introducing physics, Counter-Strike: Source is a must-have for every action gamer.
Counter-Strike: Global Offensive
Counter-Strike: Global Offensive (CS: GO) will expand upon the team-based action gameplay that it pioneered when it was launched 12 years ago."
Another one bites de_dust