One has to be vigilant and patient to find them. I just won one on Ebay for $49 shipped with keyboard, mouse, and the mac mini as well as an external HDD. It is also worth looking in the "For Parts/Not Working" section of Ebay as people sell machines with some very minor issue in that category as they have no clue how to fix the machine (or how to google search a fix apparently).
I have found Craigslist to be good for iMac G3 and G4, but anything else people seem to expect WAY to much for. In my area Power Mac G4 and G5 systems routinely are posted for well over $100, the same goes for iBook, but iMac G3's can be had for a few bucks.