Two gold coins. It was a bit of a surprise because I went to the deposit box with a regular white bone in my possession and it spat out a gold coin; I'd been expecting silver, but I also expected the shop to allow me to pay with inexact change. Alas!
Fortunately another trip to and from for a white bone got me the silver coin I needed to buy the bass (and for what it's worth, afterwards I had a single gold coin in my possession, so that much at least works properly).