I'm sure the parts are worth more than $100 easy. There is a small market just for the G5 tower case for Mods and those can fetch about $100 each. Then there is the possibility of a fully working unit by moving the PSU from the turn on/off unit to the dead unit or switching RAM or maybe it needs a new PRAM battery? Lots of possibilities and of course if that does not work, there is the RAM, HD, CPU, optical drive, and other bits that you can sell off.

I'd say go for it.