Keep the bike long enough and it might be worth something...to your great grandkids.
Where are you getting those numbers? An 8,000 unit discrepancy is astonishing if there really were only 13,000 produced.
Honda probably has the figures as to how many, of any year, came out of the factory, but I doubt there's any way to figure how many remain on the road. And for collector's purposes you'd have to figure in condition.
These motorcycles were quite expensive from new so my theory is that many are therefore present - it is harder to discard something you had to make an effort to get!
Here in Europe, a GW1000 was about $ 1600 more expensive than a Kawasaki Z900 from new. Today, a 1975 MINT GW1000 costs about $ 10,000 and a MINT Z900 can fetch $ 16,000 or more.
This must be because there are significantly more GW1000 that have survived than there are Z900 ??