The Grand Jubile was in the high middle of the Motobecane lineup, below only Team Champion, Le Champion, and Grand Record. I've generally seen the 531 Grand Jubilees sell on ebay in the $350-450 range depending on condition and components (Vitus seems to be far less desirable). It sounds like yours is all original (I've seen the 1976 and 1976 catalogs, but not 1977 specifically) which is definitely a plus, but I think that the Grand Jubile with Huret Jubilee components might be more desirable.
I picked up a 531 Grand Jubilee with Huret Jubilee components in nearly NOS condition for $200 and thought I got a hell of a deal.
You could try craigslist, but I seem to remember an original owner, nice condition Grand Jubile on the Philly CL sitting for a while with a $250ish pricetag. I would think you'd be more likely to get top dollar on ebay.