a4000 sold on Ebay for $1,225????

[psxphill]

I can get an SGI Origin 2000 with over a 1000 times the power for that price.

You could buy a modern PC for less than that, you can't really compare power and price. I imagine that demand for an SGI Origin 2000 is pretty low these days, very few people would be bidding on one to try to recapture their youth.
 
It sounds to me like a reasonable price, in the UK a broken one will go for that. We also have http://www.ebay.co.uk/itm/Commodore-Amiga-4000-Case-PX-With-Your-Old-Amiga-4000-Case-/171353070877?pt=UK_VintageComputing_RL&hash=item27e5705d1d