While my dad was in China a mechanic told my mom on a routine state inspection that she had a cracked cylinder head and it needed something like $2000 worth or work and parts. My dad told my then 12 year old self to ask the mechanic where the crack was and how it was affecting the compression and in which cylinders. He was a bit taken back by being confronted by a 6th grader, and had tried to lie to me. My mother, a natural born lie detector, then berated him for lying to a child. Thus my distrust of mechanics began.
Yeah. The main issue when it comes to cost is really paying for the mechanics labor. It's exspensive. Parts are cheap in comparison, and are generally the same price when you buy them yourself. Personally, $30+ an hour to change brake pads for 2 hours is not worth it, especially since I'd probably have to sit there for 2 hours waiting anyways, so the opportunity cost of my 2 hours is reduced to nil. I'll save $60 and spend the 2 hours myself on saturday. But to each his own.