well, its all relative...some computer stores charge $30 for a diagnostic alone, without repairs or anything! other places offer a system "clean up" for around $40 bucks on the low end...
like i said before it all depends on the time you spend, so if your looking to give the person an estimate, ask them either what they want built, or if they need purging, ask how much the computer is ****ed up and go from there.
if you want personal advise, i can tell you to build a system i would charge no less than 20 an hour...its specialty work. not many can do it
hope i could help