The inscription 'Libertad en la ley' means 'Liberty in the law,' a worthy motto for the newly formed Mexican Republic. Mexico (Republica Mexicana) issued gold and silver coins using the old reales and escudo denominations until 1870. Decimal denominations centavos and pesos started appearing around 1863.
Here are some statistics for the old gold escudos:
1/2 ESCUDO: 14 mm diameter, 0.048 troy ounces gold
1 ESCUDO: 18 mm diameter, 0.095 troy ounces gold
2 ESCUDOS: 23 mm diameter, 0.190 troy ounces gold
4 ESCUDOS: 30 mm diameter, 0.381 troy ounces gold
8 ESCUDOS: 38 mm diameter, 0.762 troy ounces gold
To evaluate these coins, the first order of business is to figure the value of their raw gold content. Do this by going to a web site such as kitco.com and finding the current price of gold. Right now it is $1368 US dollars per troy ounce, but tomorrow it will be different. Look it up!
Once you know the value of gold, multiply it by the troy ounces in your coin. For instance, if you have a 4 escudo, multiply by 0.381 like this: 0.381 x 1368 = $521 US dollars gold value.
Assuming your coin is in decent shape, without holes, cleanings, scrapes, gouges and the like, there is additional value over the gold value. This collector premium comes from the demand coin collectors produce when they want to add a specimen to their collections.
Here are some approximate collector premiums for these old escudo gold pieces:
worn: $10 US dollars added to basic gold value
worn: $20 US dollars
worn: $40 US dollars
worn: $60 US dollars
worn: $80 US dollars
These are very approximate values and can be low or high (mostly low) depending on many factors. Avid collectors of Mexican coins search for rare mint marks and special initials on the coins which make them more valuable.
If you have a nice coin in good condition, it would be wise to consider authentication, grading, and encapsulation by one of these services: PCGS, NGC, ICG, ANACS. Look them up on the Internet. Do not use other services.