Building your own or "getting someone to do it for you" is very like car maintenance - how many corners do you want to cut, and how big a risk do you want to stand? There are plenty of "backyard" people out there who can assemble a computer - but I wouldn't let them near one of mine! Unless such people do assembling for a living, or do "private" assembling regularly, they are unlikely to truly know what they are doing. And there are plenty of ways to stuff up building a PC.
Also, if one shops around, one can invariably get stuff cheaper. But there's 2 thoughts there: are you comparing exactly the same components / brands? What is after-sales support / warranty going to be like - how many places do you want to have to buy from to cobble a system together?
A final thought: how much time and effort has the person you've been dealing with for pricing put in so far? It's rude to get someone to do a whole pile of legwork for you, then take the info and go somewhere else. When I smell a client like that, I tell them - politely - that I'm not interested in having them as a client any longer (I've had to do that 4 or 5 times) - such people are time-vampires with no return on investment.
The short of it all is this: if you want to go cheap, don't go to stores or specialists, and then be happy with whatever cobbled-together system you end up with, plus be prepared to spend time with problems, issues and possibly little warranty support beyond manufacturer's basic warranty. If you want a good job done, have someone you can call if you have probs, have a single place you can go back to for warranty, and all those sorts of things that people expect from a "store", then it's only fair that you pay them what they deem is reasonable for being in business.
They're not ripping you off, they are providing you with a service and a solution.
Now that im certified i do my own stuff and my mates/family stuff although after a while you get stuck with the stereotype of if its broken ask him to fix it 
