A ducat’s weight is roughly 3.5 grams, or .11 troy ounces of gold weight. Ducats were produced in high purity gold, 23 3/4 carats, making them among the highest purity gold coins ever issued for circulation.
As of Jan 12, 2017, a gram of 24K (pure, .999+) is priced at $38.35. One ducat then costs $134.225.
So 400,000 ducats is roughly $53,690 at today’s gold price.
Ducats were gold coins, and also units of account, formerly used in most European countries; including Holland, Russia, Austria, and Sweden. The value of a ducat was generally about 9s. 4d, that is nine shillings and fourpence.
There was also an Italian ducat silver coin , value about 3s. 6d.
According to The Oxford English Dictionary, the first Ducat was a silver coin issued in 1140 by Roger II of Sicily, as Duke of Apulia.
In 1284, the first gold ducat, also called a zecchino d'oro, was struck at Venice under the doge John Dandolo. This coin, worth about 9s., bears on one side figures of St. Mark and the Doge, and on the other a figure of Christ with the legend 'Sit tibi Christe datus quem tu regis iste ducatus'; this, though it did not originate, may have contributed to spread the name, which was subsequently applied to the gold coins of various European countries.
See Austrian 1915 One Ducat Fact File to know more.