As I stated the badge is easily obtained
http://www.ebay.com.au/itm/HOLDEN-MO...item5d440566a2

Sure it's reproduction but this insert fits any of the horn caps that typically have the GTS, SS, or GTR insert (which you can also buy new).

If it is a real original one then it adds value to the steering wheel. A real good 5mm wheel secondhand is about $300 to $400 and with a genuine Sandman horn button you could add an extra $200 to $300.
But as already stated would have to be really very good.

New old stock Bridgestone steering wheels pull just over the $1,000 mark depending on the day.