In the text of "A narrative of the expedition of Hernando de Soto into Florida published at Evora in 1557" the anonymous author states that de Soto "... gathered a hundred and four score thousand ducats together..." from his adventures in Peru with Pizarro.
I've been doing a bit of google research, but I can't find anything that approximates that value of a Spanish ducat at that time. Also, the text makes reference to Rials, so I would also be interested in that valuation as well.
I understand there is no way to provide an exact number. I'm looking for an approximate number: Is there any historical evidence that a Spanish ducat of that era must contain X weight of gold (or silver)?