I am a mechanic myself and have never had my car in a mechanic shop. Once you know how to do most repairs by yourself, they all seem like a scam. Labor charges can be manipulated into more than they are supposed to be. At the mechanic shop I used to work at I was forced to charge a customer hundreds of dollars more than what I should have.
Use the internet to research how much certain jobs should cost and make sure your bill is similar. Also it helps to ask how much the job will cost before you send your car in. If they say its hard to tell until after the job is finished, they're lying. Most of the time you can accurately say how much a certain job will cost.