I've been doing this to some extent for 2 yrs now. I advertise in a church paper, where almost all my jobs have come from. I advertised in the local weekly paper for four weeks, and got not ONE call--and this was in the fall, when there should have been jobs.
For a lot of reasons, I figure I need to charge a minimum of $50 per hour to give me any wages/profit. Rather than tell the customer "I charge $50 an hour" and then have them look at their watch every time I stop to take a drink of water, I bid the jobs for a fixed price. I use $50/hour and try to figure how long it will take me, including load and travel time. Then I add an hour or two for bad estimating and added little things the customer wants. The customer hears one price for the job, take it or leave it. I don't need practice putting hours on my tractor.
The exception is neighbors, I just do it to be neighborly. Most of those jobs are an hour or less, and I chalk that up to goodwill.
If you think you are charging too much, bring a plumber or electrician to your house--or a tractor mechanic! Then your $50-75 per hour looks like a bargain--especially when you consider that most of us have $10-30K worth of equipment we're trying to pay for!
At first, I was reluctant to bid a high number, but I got over that really quickly.
Hope this helps.
Ron