I agree with the other posters that it's hard to say for sure whether the mechanic is good/bad or ripping you off. All I can say is that it's much better to learn to work on your own bike. Saves a lot of $$ and headaches from wondering if you can trust your mechanic and may save your bike from damage as you have experienced with the derailleur which could have destroyed your derailleur and wheel as well. This is usually just a simple adjustment. If you consider the time spent taking a bike to the shop most maintenance can be done in the same amount of time at home. Nothing wrong with having a pro do your work but it's not always the easiest way. Plenty of videos, articles and this forum to help you learn. Also look for a bike Co-Op near you where for a small donation (usually) they can provide you with tools and assistance to help you learn.