If I have $100 to pay someone in the US, about 25% of that will go to "fringe" (taxes, health insurance, unemployment/worker's comp, 401k match, PTO/sick, etc) and that percentage is probably higher for lower-paid employees. So, start with $100, fringe expenses take $25-30, agency middleman profit/overhead/flex-risk takes maybe 20% ($20), and the end worker gets $50-55 out of the $100 the original company is paying for them.

That the end worker is getting ~50% isn't that crazy to me. If directly employed, they'd be getting only about ~65-70%. (which is less than the direct fringe ratio because someone has to fade the PTO/sick/hiring lag flex to ensure the trash cans get emptied consistently and the agency is doing that in the other case).

While I'm hesitant to give in to my inner cynic, the anecdote about walking into the local, on-site IT in the post about his first day at google linked in the first post, especially stood out in my mind. Someone's manning that walk-in station - do they have the same benefit as the engineers? Half of which allegedly have PhD's, mind you.