I calculated my work at $150/hr, machine plus attachment. They don't clean up as quickly as you see on their videos, I generally made 3 passes to get where I wanted, both with grading and stone removal. Around here most lawns are 1 to 2 acres, I could prep them in a day and a half. Make a deep pass, about 4" to grade and bring the rocks to the top. Then make another pass another about an inch deep to grade again and get most of the rocks cleaned off. Then make a final pass just skimming the top of the soil, you should just see s little light under the rotating bar. This will get the last of the stones up and leave a finish you'll hardly have to touch before seeding.
I found it more productive to keep the end boxes on rather than try to windrow, it just seems to work better.