not an intel mini; like kaido said, a G4 mini for that price is do-able.
if you want to mess around w/OS X on the cheap, buy a used dual 533 G4, or dual 800, or dual 867, whatever you can find in your price range.
just make sure it has a decent amount of RAM. 768 MB will get you started, 1 GB+ makes it better.
you could even get a 500 MHz iMac G3 for under $150 and it will give you an idea of OS X.
i guess maybe it's the small form factor you are looking for... don't forget the Intel minis have a shared graphics set, while the G4 has a dedicated VRAM.
the last G4 mini w/64 MB VRAM is the one you might want to look for.
http://www.everymac.com/system...s/mac_mini_g4_1.5.html