Simple answer is drop that property. I wouldn't even attempt to raise the price
$140 for 6 hours of work is ridiculous. Also, you mentioned rocks and debris everywhere. The likelyhood of you throwing a rock or something else through a window or into a vehicle goes way up.
We had a similar property but it was much smaller. The whole yard was a hill with a steeper decline towards the front of the property. Shale rock all over the place. We cut it 2 times then the owner came out talking to us about her landlord cutting one time and breaking her car window and not wanting to pay for it. That was the last time we cut her property