I'm guessing you live in a relatively poor area?

I worked for a university's IT dept and we had a support contract with the manufacturer (HP in our case) and we'd upgrade at the end of that contract. We'd also have a common image that gets loaded on every morning, so every computer got reset every day. I assume that's how most campuses work. We'd usually run the oldest supported version of windows, because upgrading meant we'd have to rebuild the image and reverify all of our software, and we were lazy.

So every year, there would be a bunch of surplus computers, all 3-5 years old, and they'd usually go for $50, keyboard were usually $10, mice ~$5, and monitors varied (IIRC, we didn't replace them as often). They were usually pretty crappy computers though.