You can do this, with online retailers selling barebones laptop chassis and processors, SODIMM ram, hard drives etc...
However as mentioned above this is very tricky, but if you dedicate enough time and research, you can save alot of money for what you get.
The main thing is, you will not have as sleek or ergonomic a design as the big name laptops, as they use very complex engineering techniques and skills to design their slim notebooks.
Have you considered those websites where you choose your parts and have it made for you? That would probably strike the best balance between price and effort in my opinion, as you won't have to worry about breaking anything or having build issues, but will also be able to customise later should you require it.