I had a tech who worked side by side with me. I taught him how I charge customers on everything. He thought he could do better by charging a lot less but instead it almost put him out of business until he called me up to ask what he was doing wrong. I told him to charge more and don't go by the retail price of the part but to double the wholesale cost of the part to the customer. Let the customer know up front or even over the phone if necessary what it may cost to get it fixed. That way, you don't waste your time or the customer's time whether they want to spend the money to repair the unit.
The way I work is this:
Diagnostic Service Call Charge $80.00. If the customer asks if this charge is blended into the repair, I respond, "No, it is a separate charge for me to disassemble the machine and determine what is wrong with it. Once I find the problem, then I will prepare an estimate to repair the unit. It will include parts(if any), labor and the service call charge. If you decline the estimate, I will put the machine back together and only charge you $80.00." Even if I walk up to the machine, turn it on and determined the problem within 2 minutes, I still charge $80.00.
Parts. I always charge double what I paid for the part. If the part costs me $5, I charge a minimum $40.00 for the part. If the part costs me $250.00, I will charge the customer $500.00 for the part. If the customer says that they can get the part for $250.00 online, then I would refuse the job because I won't be able to guarantee the job. Also, I tell my customers that the sales tax is already included in the price of the part so all of my bids are rounded to the nearest $20.
Labor. I always give a flat rate estimate for all jobs but I usually base it on $80.00/hr. So if I can do this job in 1/2 hour, I would charge only $40.00 labor. If I know this will take me longer than an hour, I usually guesstimate around $80 to $120.00 labor for the job and stick by that. I won't charge any more unless I find another problem along the way.
If the customer ask how long is the job guaranteed for, I just tell them "Usually 30 days but I want you to call me back even if it's after 30 days if it's the same problem. I'm here to help."
People sometimes ask why I'm so expensive and I just tell them, "I'm worth it". I have found that the $80.00 service call is the only way to filter out all of the cheapskates and stingy people. They're the ones who will go after the "Craigslist" repairman and that's alright by me. My phone keeps ringing each and every day so I'm either booked solid for a whole week or I'm available for same day service. There has never been a day where I had nothing to do. Sometimes, I will even get called out on Thanksgiving but that's because I'm such a nice guy.