I know nothing about plumbing. So when my kitchen sink went haywire, I called a plumber. He installed a new waste disposal for me and, according to the Internet price listings for the exact model he put in, overcharged me by about $250. When I implied I knew what was going on, he explained it was for warranties, which... seemed implausible unless he was getting stiffed himself.
I decided not to take him to task for it, since he was probably giving me the price his company told him to, but it was still irritating and something I'd like to avoid repeating.
Parts and supplies (whether for plumbing, auto repairs, etc.) are always marked up about double what you would pay retail yourself. You can buy a water pump for a Chevy Impala for about $50 online. A competent shop will most likely charge you $100 or more, plus labor to install it, but they know it's the right one, they know what other gaskets, seals, etc. they need to replace along with it, they have the tools and knowledge to install it properly, and they will warranty the job and the part against defects for some period of time. That's what you're paying for when you pay a shop or a professional to do something for you.
If you want to take that on yourself (and it's not really difficult, fundamentally) you can save that markup and the shop's labor charges. I used to do this a lot more than I do now, because even though I enjoy working with tools and fixing mechanical things, when I compare spending 3 hours replacing a water pump on my car to the opportunity cost I very often decide its not worth it (this has changed as I have grown older and my free time has become more valuable to me).
When I was in college I used to work on my own cars, these days however I am time poor and money rich. For me it has made more sense to go out and buy a brand new car that I can drop off at the dealership for warranty work or anything along those lines, rather than spending 4 - 5 hours dealing with whatever issue has cropped up.
It's a something you have to weigh off, and decide yourself. I'd rather pay someone to work on my car now and be able to spend that time working on projects and making money than work on my car and spend those hours on that (although I do miss it tremendously, that's what getting a project car is for though ;-)).
I would certainly hope that he charges the NewEgg/Amazon price for the part in addition to a healthy hourly wage, and transport costs if there are any.
If he's keeping a stock of a lot of parts it can be worth a markup. But in general the cost of having someone buy a part and install it vs. buying the part yourself and paying someone to install it should be pretty close.
I've never actually needed repairs to my own computer. I live in a very strange vacuum where things tend to not-fail around me. I've mostly chalked it up to the "developer looking over your shoulder" effect.
I decided not to take him to task for it, since he was probably giving me the price his company told him to, but it was still irritating and something I'd like to avoid repeating.