Doubt it's much cheaper. Custom PC had a feature on it a few months back, might be worth having a look around their site to see if there's anything on there if you don't have the mag. The way to go about it is buy a bare bones chassis which consists of case, mobo, screen, battery, charger, kb and touchpad etc. Then the CPU, RAM, HDD and graphics module are bought and installed separately. So not *that* much in the way of customisation that you can do really, imo, since most prebuilts have that much and more customisation available at buy time (e.g. Dell). Other advantage of prebuilts is that they are generally much better integrated if you know what I mean (i.e. nice function key combinations for control of bluetooth and wifi, that sort of thing).
Still, if that hasn't put you off, I think Acer for one make the above mentioned bare bones chassis, if you want I can have a look at the CPC article for the others.
Hope this helps, null