Nick is absolutely right, the prices some sellers are asking are far too high for what you're getting.
Mac Mad, are you in Devon, UK? If so, very few G4 Cubes even made it to the UK in the first place, but they do pop up on eBay UK from time to time. I bought mine in January last year, and there are always one or two on at any given time.
If you want it in full, working order, you'll have to be patient, and be willing to possibly pay a price that is worth more than the sum of its parts!
It is a cracking computer though. I love the look of it on my desk
Good luck!