Where I am (Seattle), such a rig could be had for next to nothing. We have recycled PC places that sell towers for under $200 that meet/exceed that. Hell, I’ve given away at least for systems like that!
Craigslist and Ebay can also find you such a system; I got one from Ebay for $160 when I lived someplace that didn’t have the types of stores Seattle has. I’ve also got a few systems for free from friends who recently upgraded.
In each case, I had to use the monitor, keyboard, and mouse I already had, but those items are cheap. For instance, I got a 20” CRT monitor from Craigslist for free, and my cordless Logitech mouse was $6.