I have a knife sharpening business, many of the knives I get look like they have been dragged behind a car for a few miles before i get them.
You need to be completely comfortable with your sharpening skills before charging someone, then, as Ken says, you need to treat each knife as if it were a family heirloom, don't look down on a knife that cost $6.95. The owners mother may have given it to them and it may have special meaning, every knife is important and you need to be grateful that people trust you with their knives.
The folks who own 20 dollar knives may consider them as their their prized possessions and regardless of how much they are worth, you need to treat them as such.
Cutco knives are best in the world, I learned this quickly from Cutco owners, you should consider it a privilege if you get the opportunity to sharpen them
You need to be comfortable with knife repairs, it is part of knife sharpening. You get a knife or a bundle of knives, inspect the edges and some will have nicks in them, you need to repair them before sharpening them, you will find this very rewarding and it isn't hard to do.
Know that for many people, paying to get the knives sharpened is an expense and often comes after the wife has nagged the husband into letting someone do it, prepare to be judged.
I charge $1.25 an inch, repairs are normally free.
In my case, on day one, I took a few of my own knives that I had just sharpened to the Manager of a high end kitchen store here, (after talking to him first about sharpening) once he saw the knives, he advertised for me. That was the key, establishing contact and proving myself to a person who knew knives.