I work for a retail guy. $150 service call for one hour, $100 hour labor. Big jobs are bid out instead of on time. 90-day warranty on the parts/work we did. We've been in business for 17 years, but I've only been with them for about 7.
But we are pretty expensive as far as fix-it places go.
If you have the knowledge, parts, and time to do it, then start at $50-75/hour + parts. Or set-up a $35 show-up fee, look at the game to determine what is wrong, then bid it out ($75 to recap/rebuild a monitor - including parts if it a normal 4900 or something). if you're confident in your repair skills, offer a limited warranty on the work you do.
I say to charge a show-up fee or minimum service call, because otherwise you'll be driving all over, just to have people tell you they'll "talk to the wife" about it.
Just make sure you only accept work you have time for. Some things will be easier to fix at your place rather than theirs - as you may not always have the part on hand or don't want to be recapping a monitor on their gameroom floor or kitchen table - so you have to take into account multiple trips and deadlines, plus space and time at home to do it.
I had a friend who tried to do it as a part-time "extra money" thing outside his real job. He had to stop when he had 4 separate games at the same time with issues he was having problems fixing, and a wife who was complaining about all the time he was spending with them...