The site has enabled software vendors to slack off on their documentation. If there was no StackOverflow the vendors would have to do better. I'm thinking of msdn as an example since a lot of the Stack questions are MS technology based. MS is far from alone though. Much of the money saved has gone to vendors.
Even given those points it has still saved a lot of money. But billions? I don't know if it would be that much. It would be interesting to know what the global number for reputation points is. There may be a correlation to money saved.