High street retailers may offer a cheaper price because most of their PCs are assembly line built. Like HP, or Packard Bell, Compaq etc... They also appear to use proprietary components which means you can - generally - only upgrade with their parts (which cost more). The one man band probably uses generic parts which can be bought anywhere. For a quick look at a one man band click here
Alternatively, click here
Most "systems" come without a monitor.