Well, it is possible. After all the big boys (IBM, Dell, and the rest) are doing just that -- they just get the parts from different manufacturers and assemble them. Now since there is not simply enough demand and availability (as Nemesis rightly pointed out), this is not being done by our local (un)friendly
computerwala.
Secondly, it is not just simple arithmetic coming to estimating its cost (based on the components). Notebooks use components that are a little different from desktops (and much more expensive). For example, they would use mobile processors, or low-power consuming ones (Centrinos). Actually, some use the desktop processors as well -- and you can almost see the smoke coming out of them
. Then you have the expensive SODIMM RAM for memory. Then the shock-proof HDDs (so that you can move about with your notebook turned on) that cost you the earth and still will go no faster than 5400 rpm!!! I wouldn't dare to think of that 7200 rpm HDD for 20K or thereabouts. Then that pretty-looking LCD screen at a price we would all like to be amnestic about.
Add it all and what do you get - an expensive toy or a portable computer, depending on the way you look at. In the end I guess you need to dig deep at your pockets to get you that bliss called "mobility."