What is the approximate value of a Spanish ducat in 1557?

Question

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)?

Answer 1

Lifting the contents of the very helpful link provided by @gvk into an answer:

Source: Fiat Money in 17th Century Castile, by François R. Velde, Federal Reserve Bank of Chicago, and Warren E. Weber, Federal Reserve Bank of Minneapolis and University of Minnesota.

Footnote 1 states:

The ducat disappeared as a coin in 1537 but remained as a unit of account, representing 375 maravedis.

Table 2 Castilian monetary system, ca. 1590 (before the onset of vellón inflation) lists the following in the column "Purchasing Power":

• 4 maravedis -- 1/2 lb bread
• 15 maravedis -- 1 bottle wine
• 50 maravedis -- 1 spring chicken
• 80 maravedis -- 1 day skilled labor
• 200 maravedis -- 1 ga. olive oil
• 350 maravedis -- 1 bushel wheat
• 1450 maravedis -- minimum weekly middle class income

So 1 ducat was the rough equivalent of 1/4 the minimum weekly middle class income. It would have bought you four and a half days of skilled labor. Or 7 (live!) chickens, a bottle of wine and a pound of bread. ¡Buen provecho!