I'd at least check google to see if there are Mac repair shops near you. Give them a call and ask for a quote to fix it. Might as well get a 2nd opinion.
But yeah, if it's gonna be that much, just scrap it.
You can salvage the hard drives, at least, unless you're looking to fill the MacPro up with 2 TB and don't need your old drives.
I bet the RAM is worth something on E-Bay too. Check Crucial.com to see what it goes for new and judge from that on how much to charge for it.
As for the casing? Well, the Mac-Aquarium guys don't seem to have a G5 on their page yet...I bet you could really blow them away!
http://www.theapplecollection.com/Collection/MacAquarium/index.shtml
Or how about a mailbox?
http://www.flickr.com/photos/digitaldust/88847257/in/pool-tuawrigs/