I'd say you have a $130-$140.00 treasure there, and that's if it is a repro --
http://www.vw-online.co.uk/acatalog/Karmann_Konnection_Flat_4_Banjo___GT_Steering_Wheels_22.html
... and a little history borrowed from 'flat4.co.jp':
FLAT-4 vintage horn buttons are reproduction of accessories from 50's and specially"Golden lady" a.k.a. "Greek lady" is known as one of the "Rarest of rare" items among vintage enthusiasts, and also the rare "SAN & MOON" is konwn to be seen with Rometsch and EMPI Formular steering. These horn buttons are often associated with Porsche 356A, Rometsch, Hebmuller, Dannenhauer & Stauss, and other high class coachbuilder. We also produced "St. CHRISTOPHER" horn button to join our line of horn buttons. All these horn buttons are made as well to fit not only FLAT-4 banjo steering but original PETORI steering wheel, and quality of workmanship is there to compete with originals.
( http://www.flat4.co.jp/f4original/f4-banjo/emain.html )