Having seen H1B document submitted to the DoL, I know that firms misrepresent these numbers. An H1B is granted to immigrants for positions that ostensibly couldn't be filled by US citizens, this is rarely the case. Firms have to show that a job application was life for a certain amount of time and they were unable to fill it.
Finally they need to claim that because the position is so hard to fill, they are willing to pay above prevailing wage. See H1B LCA. http://en.wikipedia.org/wiki/H-1B_visa#Labor_Condition_Appli...
They however do not need to prove that the claimed wage is what is actually being paid. These numbers are thus overestimates and I would guess >10% off the mark.
That said, it is obviously illegal to misrepresent these numbers and some firms are more scrupulous than others.