I think that headbadge started in 1976, I had a 75 Grand Jubilee and it has the older flat aluminum "M" headbadge.
The problem is that all the best parts are gone. That mark is likely just a wear mark from a bag bracket or basket but I did see a GJ crack around the top lug once, but the crack followed the outline of the lug and the bike had a ton of miles on it by the time it failed.
Price wise I think its way high, between the unknown of that rusty mark and the missing original parts, I'd say $200 tops if you really have to have it.
Someone probably sold off the original parts on fleabay years ago and sold what was left locally.