A perfect restoration usually garners top-dollar as compared to perfect low-mile unrestored, which is just about impossible to find anymore. Nice originals are cool to enthusiasts but the buyers w/ the really big bucks don't want to or have time or would actually do a complete correct restoration. They want a finished piece in eyeball-popping concours condition thats museum-quality and will win any show.
Investment? Not all motorcycles are investments that will go up, hold, or not lose value. CBX's, RC30's, Vincents, and sandcast CB750's are a few on that short "A" list.
Vic World was selling his sandcasts 2 years ago for $22,500; that may be what that museum paid then. His price is now $29,500. And he's running out of engines, frames and stock.
Spending over $10,000 on a professional sandcast restoration is ez, and 100-200 hours labor. From a business viewpoint, a % mark-up on parts is needed, plus X amount $/ hour labor.