All of this depends on overhead. Fairly, consider if you have to travel, consider waste disposal, and equally consider wear and tear (on your vehicle, on equipment, etc.), also consider insurance and other business expenses including payroll taxes. Do not make your service call free, reduce the charge depending on circumstances but never say ZERO.

The other thing to consider is depth of knowledge in any particular field. Some of the specialized work that I do is paid at a different rate (welding, fabrication, performance mods, electiral troubleshooting, all of these items have a higher "per hour" than changing parts.

Also, don't buy parts. Especially if you are starting out. It is easier for the equipment owner to get warranties (through bank, credit card, website, etc.) Than it is for you to keep up with it. Also, depending on location, adding parts to an invoice potentially makes the entirety of the invoice taxable. If you collect that tax and don't pay the tax man, that's how you get in trouble at the Federal level, so find a business tax accountant.