There are two basic models for customer relations: adversarial and cooperative.
In the adversarial model, the customer is a bag of money. The business wants to get as much money as possible with the least effort. The customer, knowing this, wants to avoid paying. It becomes a contest between gouging business and chiseling customer.
In the gouging model, work quality only serves to keep the customer calm, quiet, reassured. The reassurance may be false. Who cares? Get the dough. Get the mark out the door. Take five minutes, spray the bike with aromatic lubricant, knock the worst of the dirt off, call it a tuneup and charge $50.
If you don't really like people and have really fallen into that "us versus them" mentality, conning customers seems perfectly legitimate. They're not clients. They're prey.
Some customers are easy to hate. But not every poor-mouthing chronic chiseler deserves to be screwed. Some of them can be educated to understand the value they're receiving for the higher price. Others are simply playing the business game, trying to drive a hard bargain because they enjoy the haggling. Feel free to laugh in their face. Don't play the numbers game with them. The chiseler always wins that one.
In the cooperative model, even chiselers can be accommodated. Perhaps you have to do it by showing some of them the exit, but don't waste time being hostile. Just be courteously unavailable.
The cooperative model seems like more work at first, because you actually have to perform the services you say you offer. But I've noticed over the years that trying to get out of work ends up being more work than actually working. Once you accept that the work needs to be done, you can get down to doing it as quickly as a good job allows. Then it's done, usually just once, correctly, and it's out of your hair.
The guy who fixes my car was talking to me on the telephone earlier this summer. We both had heavy repair loads.
"Well, work too hard," said Rich, as we got ready to hang up.
As so often happens when I talk to Rich, I took it to heart. Do good work. You don't always get to choose when it comes to you. If you've created a trusting customer base, you really do owe them the expertise you've trained them to accept. Do less than that and you have to start all over again.