I could see an ARM board for about $150.
Andreas, myself, and many others have been discussing ARM for a while.

Bur I keep coming back to the PPCs that Andreas has mentioned because they look like good candidates for a less complicated upgrade (if they can be obtained at even remotely affordable prices)

I've been thinking about ARM as a way to set up a low cost low power draw Ubuntu system. But I don't want to lose the ability to run PPC software.