The 1758 and 1759 are common dates for the Ferdinand VI 8 escudo
1793 is a common date for Carlos IV
Is the 1812 the large bust or small bust? Large bust is a premium coin, both styles are 2 yr only types in high demand.
For price estimates, I would compare them to coins listed for sale on a website like Steinberg:
http://www.steinbergs.com/SouthAmerica.htmlor another coin company specializing in certified gold coins. That will give you the most accurate numbers because these coins fluctuate with the gold price and currently are in very high demand.