I always price mine by the job, and I always look at them first.
My two main competors use a 27 HP tractor with a 5 ft mower, the other uses a 40 HP tractor with a 6 ft mower. I have a 56 HP machine with an 8 ft mower. When customers ask how much I charged by the hour I always lost the jobs to my competors because they were lower. People didn't understand I could mow a given amount of acerage quicker, thus having less hours paid. People also didn't like to have the clock start when you loaded the machine at your house. They felt like their were getting cheated.
I also look at the job first. Seems like everbodys defination of "knee" high is different. That could mean from 1 to 3 feet high. Also be careful with what type of land you are mowing. I had a guy once who wanted 5 acres of old farmland cut. To me "old farmland" means a cotton, or soybean field that wasn't planted for several years and had grown up with brush and saplings. This guy meant an old farm place that was basically abadoned 5 years earlier. It had cars, trucks, tractors, implements, barrels, etc everywhere buried under 5 years of grass, vines, and sapling. It even had saplings growing thru some of these items. The ole fellow who bought the placed wanted me to clear all that stuff out at a normal "bush hogging" rate. I passed on that job.
I basicaly look at the jobs and take several factors into account; grass type, grass height, terrain, obstacles, trash, travel distance, will they be a repeat customer, etc. Then I use a reference number of $40 to $45 per acre if it were a perfect situation. I add anywhere from $2 to $15 an acre depending on the other factors i just listed.. So when I give them a price it's an exact number that will not change. I will never break down how much I'm charging for loading, travel, trash removal, etc. I give them one solid number. Most people really seem to like this knowing exactly how much it will cost.
This approach seems to work well for me. I have probably 15 to 20 customers where I cut their fields 3 to 4 times a year for the past 4 years.
Something else I do to pick up more work is what I call "grouping". If I have several customers who have really small pieces of land they want mowed where it's just not ecomically feasiable for me to load up a tractor and do their one job I will group them.
So what I will do is mow all these small jobs on one day. They all must be in the same area, or it's not worth my time. In other words I may be passing Joe's and Sam's place to get to Dave's place. So I will do all 3 on the same day. The customer's must understand and agree you will cut their place on your terms. You pick the day and time you will do the work. They all seem to like this because they are not having to pay a higer rate for your travel time. This last summer I had one "grouping" day where I did 5 small jobs and made $1,000.00.
Good Luck....