That's a major coin, Stefan. Nice!
The one in our picture comes from Stack's Bowers, a world leader in high-end numismatic (coin collector) material. It is from an auction by numismatist Rich Ponterio where is sold (are you ready?) for $1600 US dollars.
The Ponterio coin is a gorgeous example. It has been authenticated, graded, and encapsulated in a numismatic slab, and you should consider this for yourself. Look for the following companies on the Internet: PCGS, NGC, ANACS, and ICG. Do not use other companies.
The world coin catalogs list the following catalog values for these gold ducats:
average circulated: $250
well preserved: $450
fully uncirculated: $850
The fact that the pictured coin sold for $1600 shows that catalogs are simply guides to coin value, not final authorities.
CoinQuest thanks Stack' Bowers and Ponterio for use of their coin image.