I put together ("build: would suggest more to the effort than there ever really is) a PC for the first time around 1996, and before that I happily installed modifications and upgrades to my Amiga. Before that, dating back to 1980 when I first had a personal computer (Z-80 based), I opened them up for a look, but never did anything to them.
I put together dozens for friends and family and myself over the years - perhaps over a 100 in all. I long ago gave up doing initial "builds", though. It's just not worth my time and effort compared to having someone else do it (although the rates around here are about $100- on top of the components).
There are always options to do it cheaper, as with almost anything in life, but it's the after sales support and service that costs you money and, ultimately, is well worth it.
I tend to agree with Bart, although less emphatically :-), that for someone who hasn't done it before and isn't up with the latest info or have a technical background, it can be quite a drama (it can also go surprisingly well, as I've seen many times) - but it's always a risk.
For a commercial/business use, it's almost always worth paying a little extra for piece of mind - at least at the beginning.