There actually is kind of a history with betting and CS; the meme "bet t all" and a picture of a swat policeman petting a cat references this.
Basically pre-round or mid round you could bet some of your in game currency on a side winning. This let you recoup lost money which you could use to buy guns etc. the next round.
I always thought it was just a fun addition to the game but if you did it real time, round by round, with real money it could be cool too.
Of course, finding the way to do it legally (in the US at least) is the big challenge.