i am a G26 owner..
I would agree a Good Sailable boat usually is about $4000 - $5000
A kick butt fully decked out boat with whistles etc.. $9000
That being said.. I saw an ad a week ago for a fellow giving away an Oday 22 with furling and in the water good etc.. just needed it gone, and the hassle of selling for the $$ was not worth it.. go figure. (needed a cradle)
$500.. consider that a hull - is your stding rig complete? if so .. ok..
The decks usually are solid, the hull is for sure.. but I would moisture test.
In the cabin .. look for rot.. everything is wood.. so check the bulkheads and all the interior compartments..
But add sails, and outboard, fix electrical buy some batteries, and all the "sailing accessories" I am sure are missing... well the point is.. do you want to start at $500 and do it yourself?? is that FUN for you? Or do you want to pay say $5000 - $7000 and just go sailing.
I paid $3800 - and I am up to about $5000 now.. and I can go sailing.. its not perfect, but i have put a fair bit of elbow grease into mine so far.
Good luck.. There are several Grampian sites, and a Yahoo groups site that is very good.