Back in the days when I was doing contract work pricing was my big problem and I never really solved it. Some jobs I took a bath some made hay; the problem is I was bathing far more often than necessary, boy was I clean .
One thing I did start was having a minimum of half a day, around $200 back then. When a customer came in the first question was is the part worth at least $200 to them, and if the reply was no I told them I couldn't do it. I defintely lost some jobs that way but overall made more money for my time.