ExtremeTech published an article on creating a cheap $400 computer:
Speaking of Mom, her computer wasn't aging well. We (meaning my father and I) had upgraded it a few times over the years, but it was a relic with a stuffed hard drive and some bizarre problems that even the strongest spyware and virus defenses couldn't work out. I found out when I tore that sucker apart that the CPU, an ancient Athlon, had a manufacturer date of 1999. It was time for a new computer for my mother, Janice Durham.
The problem was, I was out of spare parts (I threw most of them into recent builds and upgrades for my Dad, Joel Sr., my sisters, Shawn and Robin, and my wife, Emily). Thus, Mom had to pony up the dough for her new system. Her system's eternals were fine; the monitor, speakers, keyboard, and mouse could all make the transition just fine. She needed a new case and everything that goes into it.