I own a '77 RS with the 40 mm pipes. Mine is not original by any means. But it's a bike that I truly enjoy riding.
Like all things old and collectible, the old RS's are not worth what they once were.
Also know that you probably won't be able to recoup the money you'll end up putting into it to get it running and back on the road the way it should be. Airheads are more of a labor of love than they are an investment.
I'd say if you could get it for anything under $3K, it would be a good deal. It's probably going to take $2k worth of "love" to get it back to where it needs to be. Keep that in mind.
"By the way, you don't have to be sweaty and holding a basketball to enjoy a Gatorade. You could just be...a thirsty dude! Gatorade forgets about this demographic!"