Use your hourly rate, estimate your total time for the project to
get a ball park price then get half up front to start, put all in
writing. No pencil to the paper until there is money in hand. No
drawing into their hand until the balance is paid. Get it all in
writing. I figure my design time at the same rate as my bench time.
Any changes is still time spent on the project. An hour is an hour
whether I am pushing metal or a pencil. Obviously I do not do CAD