According to some people I know (they are all on H1B) typically H1B offer is 10-15% less than average at software big corp such as Microsoft and Google.
That may not be completely true. All H1Bs I know make 10-15% more. At some point - we need to accept that US has broken education and health system. And don't have enough talent to fill all the positions. H1B is cheap - is indeed true, but that is not the fundamental problem.