Monetary Equivalents in the Late Eighteenth Century
A silver peso in colonial Mexico was worth eight silver reales and each real was worth twelve granos.
In Spain one silver peso was equivalent to twenty reales de vellón, the monetary unit most common there.
For additional details on the Spanish monetary system at that time, see Bernardo García Martínez, “El sistema monetario de los últimos años del periodo novohispano,” Historia Mexicana, xvii, 3  (1968), 349–360.
Exchange rates. From 1783:
One silver peso was equal to one silver dollar;
one silver real was worth the equivalent of twelve cents;
one grano was worth one penny.
Before 1776, there was a substantial difference in the trade value of these coins between England and its colonies. In England the Spanish milled dollar (silver peso) was worth anywhere between 4s. 3d. and 4s. 6d. st., up to 4s. 9d. st. In New York, however, the Spanish milled dollar was rated by custom at 8s., in Pennsylvania at 7s. 6d., and in Virginia it was worth 6s. 8d. by 1764. These variations were also common in other regions and markets around the world.
Generally speaking, at this time 1 silver peso was equivalent to approximately 5 francs and to 1.7 Dutch florins.
The prices of silver varied in relation to gold. The variation of silver against gold after 1760 was not trivial. The ratio of gold to silver in 1760–1769 was about 14.51:1.00.