It depends on the price bracket, to be honest.
There are lower pricepoints when you simply can't build one cheaper than a manufacturer, but as you move up in terms of performance, they start charging more for what you can get cheaper yourself, etc.
If I was going to need like a £150 rig, I'd buy pre-built, although I did make a fairly good friend's rig with £150... E4300, Gigabyte G31 board, 4GB of RAM, Hiper 425W PSU, WD 250GB AAKs, Sony optical drive, CoolerMaster elite 330 case... but I got everything used.