Couple things; my Dad is a retired mechanic, so I know that arena better than joe public does, he was in indy and dealership shops. The line mechanic is not going to make much more than an average person working any moderately skilled job and they are required to have all the tools as mentioned. The longer you are in it, the more tools you have/need. Especially if you are at an indy shop doing whatever comes in the door. At dealerships it is not any better, although they are required by the oem to have the diagnostic equipment, they are only required to have 1 of that tool, so many times the mechanic (usually on commission) is waiting in line to use the equipment and sometimes it makes sense to buy your own if you want to make a decent check at the end of the week.
As for warranty work, neither the dealership or the mechanic likes it. The oem pays pennies to have warranties done and you have to do it or lose you dealership. The mechanic gets lower pay to do them as well, so while sometimes they can do OK on them if they do a bunch in a row, they never do that well.
I have told Dad many times he should have got out of it long before he did, poor situation for the employee.
One other thing with some dealerships is the owner(s) many times gets a check directly from the oem when a new car is sold, this is not ran through the store, but sent to the owner directly, so the owner is sometimes doing better than the store and will make more off the sale than the store.
It also depends greatly on the dealer and the oem whether the repair side is keeping the store afloat or the car sales. In my experience, neither side is a good career move....
Now as far as the machining world, I think if you are competing with the big boys, you are going to be hard at it and it may not be the best decision. I am trying my best to be in areas where there isn't much if any competition. Part of that is being in a weird niche that uses both wood products and metal. Alot of times I have a different perspective on what processes to use to make something that someone that is only experienced in wood or metal won't be aware of, so I have taken advantage of this on quite a few jobs where I have used some unorthodox methods to make some parts that others weren't able to.
I think that the main reason why precision parts are cheap is because there is always someone out there that will do it cheap. That is not me, if my machines and processes can't make me money on it, I am not going to do it just to keep the machine running. I'd rather play or stay at home. If someone else can do it cheaper than me, more power to them. I can say that because I rarely have heard a customer say that they can get someone else to do it for whatever reason.