I have a hard time paying anything over $300 for any old computer.
Between $100 and $200, I'll think about it.
Below $100, or free? I'm totally there.
I guess it depends on what it is. I did see one of those "lampshade" iMac G4 800 MHz computers at We Love Macs for $499. I can't believe that such a design triumph for Apple was so short-lived, but you can never figure out Apple. Still, $499 is a lot of money. For $200, I'd jump on this in a second. For an iBook G4 in good condition, I'd pay $350, not more, but would prefer to pay $200. For any G3 iMac, I wouldn't go above $200 for a 500-700 MHz model, and for a 233 MHz tray loader? I'd have to say between free and $30 is my threshold, if I even want it to take up any physical space in my life.
Right now, The Back Room doesn't have a Mac installed. Instead, that's the home of This Old PC, which with all I'm doing with the Palm handheld these days, is destined to stay there for quite some time. I'd love to have a really nice all-in-one Mac back there, especially one that could run OS X 10.3.9, but when's that going to happen?