The ZX81, could be bought as a build-yourself kit for £49.95, or for £69.95 assembled, and proved very popular, with Sinclair producing up to 40,000 a month.
With only 1K of RAM, a very low resolution black & white display and no sound, it is far removed from computers of today.

Pictures, unfortunately the screenshots are from an emulator as I never did get a working power supply: