I think Jinman is pretty much right on target.
I believe you will find your prices going up as you get further into the business and gain more experience dealing with the customers.
I believe that even though your stuff is paid for, you need to be charging enough to replace it, or what it would cost to purchase it and operate it with a note. Otherwise, what will you do when it wears out.
I would suggest that you go outside your operating area a bit, and see if you can speak with someone who does a similar business with similar equipment that does not see you as competition and would be willing to share some experience with you.
I occasionally do this on the landscaping / lawncare forums and it helps sometimes (as you are doing here) to get an outside logic check. I am not sure if there are particular forums for the type work you are describing but I would think there would be.
While a business plan is great, excellent, needed whatever, I would first tell you that you need a "mission statement" or whatever you want to call it. It is really the heart of your business plan and it should basicly be something like,
I want this business too_______________________
maybe it is support you and the family, or maybe it is to grow to a million bucks or maybe to give you enough money to go to the ball games on the weekend. But that will give you an indication of how much you need to make.
Then start doing some math. A little backwards and forwards dividing by how many days you actually think you will have work, (and it will not be 20 days a month) then start figuring costs of your equipment (what it is worth if you had to replace it, not what you owe) and then add in what you need to make per month to support your standard of living or what your mission statement say's and I think you will see your prices go up considerably.
One of if not THE best pieces of advice I got when I started my first business was given too me by Chuck Shoop, a very experienced businessman that mentored me.
He said to always resist the temptation to start dropping your prices to get work in the shop. Especially if it get's to breaking even kind of stuff.
He said the difference was you can go to bed Tired, broke and Hungry, or just Broke and hungry. Why be tired if you are still Broke and Hungry?
Unfortunately I have not always heeded his advice and have gone to bed more than once, Tired, broke and hungry, not to mention having a sore leg for kicking myself in the butt....
The other tidbit I will also add, is when you walk in to that job and it just feel's wrong.... IT IS. Time to walk away. I don't know how many jobs we have done where we had a bad feeling about a customer and proceeded forward and wished we had stopped right up front later.
Good luck with your business, there is money to be made there.