...is cannibalizing parts from old desktops. Usually stuff like hard drives, cases, fans, power supplies, disc drives, sound cards, etc., are good candidates. And the better you know your computer, the more likely you are to know what you an and can't (or will and won't) cannibalize. Really, replacing a motherboard, video card, and processor are often enough to "replace" a desktop with a more or less new model.

In that sense, the savings of building your computer are higher if you're upgrading from an existing desktop, and later, when you need to upgrade from a desktop you built yourself.