Around 1791, a gold ducat was generally equivalent to about 2 to 3 Spanish dollars, known as "pieces of eight." At that time, a Spanish dollar was roughly equivalent to about $1.20 to $1.50 in 1791 US dollars.
Therefore, 100 ducats in 1791 would have been worth approximately $200 to $450 in 1791 US dollars. However, it's important to note that these are approximate values and the exact exchange rates may vary depending on the specific circumstances.