I'd measure it out myself to make sure, walk the property to check on level ness, how is the current condition of beds and turf. At 6 acres and level that's about 2 hrs mowing with 1 60" ztr on a single pass.
I've found the nicer the property the more you need to charge. Someone with a property that size in pristine shape is going to want it to stay that way. If its full of weeds then you might be able to mow blow go.
Often on larger properties I break my estimate up and show a price for mow blow go and a bed maintenance price so the person is not shell shocked. Many times they think....for a little bit more i can have my beds maintained.