The term Peso could be used as a general term for a "dollar" amount as a valuation of the wreck "material"
If one person was transporting a trunk of emeralds, escudos, reales and maravedis it may be stated as 5,000 pesos lost in a trunk owned by Mr. Sanchez. That doesn't mean Mr. Sanchez lost 5,000 eight reales.
But more direct to your question....
A half reale could fetch $100 which would be .6 gram
Also Pesos isn't a modern currency standard...
Peso - Wikipedia, the free encyclopedia
1537–1686 piece of eight
Main article: Spanish dollar
Peso was a name given in Spain and particularly in Spanish America to the 8-real coin or real de a ocho, a large silver coin of the type commonly known as a thaler (dollar) in Europe. It had a legal weight of 27.468 g and a millesimal fineness of 930.5 (25.561 g fine silver). This real de a ocho or peso was minted in Spain from the mid 16th century, and even more prolifically in Spanish America, in the mints of Mexico and Peru. It was originally known as the "piece of eight" in English, due to the nominal value of 8 reales.
The piece of eight became a coin of worldwide importance in the 17th century, especially in trade with India and the Far East, where it was immediately melted down. At this time, the piece of eight was produced in Mexico and Peru in a rapid and simplified manner. Instead of making a proper flan or planchet, a lump of silver of proper weight and fineness was cut off the end of a silver bar, then flattened out and impressed by a hammer. The result was a crude, temporary coin, an irregular lump of silver. This type of coin became known as a cob in English, or a macuquina in Spanish. The Crown was entitled to a fifth of all gold and silver mined, the quinto real (royal fifth), and cobs were a convenient means of handling and accounting for silver. Although intended to serve only temporarily, some did remain in circulation as currency. Because of their irregular shape and incomplete design, cobs were ideal candidates for clipping and counterfeiting.
This coin was originally known in English as a piece of eight, then as a Spanish dollar, and then as a Mexican dollar. In French, it was a piastre and in Portuguese, a pataca or patacão. The Spanish names at various times and in various places were patacón, duro, or fuerte.
