Peru issued these nice looking gold coins from 1950 to 1970. They are worth their weight in gold. Today, with gold prices high, that is not too shabby!
Your first order of business is to look up the value of gold. On Kitco.com, the current spot value is $1417 US dollars per troy ounce. Tomorrow it will be different, so look it up right now.
Next, you need to know how many troy ounces of gold are in your coin. That is where catalogs come in. (This gold coin, like many modern gold coins, shows the gold content explicitly in the inscriptions, in this case in grams, not troy ounces. Without inscriptions, a catalog like The Standard Catalog of World Coins will have the gold weight directly in troy ounces.)
5 CINCO SOLES: 14 mm diameter, 0.068 troy ounces gold
10 DIEZ SOLES: 18 mm diameter, 0.135 troy ounces gold
20 VEINTE SOLES: 23 mm diameter, 0.271 troy ounces gold
50 CINCUENTA SOLES: 28 mm diameter, 0.677 troy ounces gold
100 CIEN SOLES: 36 mm diameter, 1.354 troy ounces gold
All that's left is to multiply. For Cindy's 50 (cincuenta) soles coin, today's value is 0.677 x 1417 = $959 US dollars.
Since Cindy's coin is mounted in jewelry, the $959 value is the actual value. If the coin was not mounted in jewelry and was mounted instead in a numismatic slab with high grade such as uncirculated MS70, a collector might pay $10 to $50 more than the gold value to get a superb specimen for his or her collection. Without high grade, the collector premium does not apply.
Finally, if you buy or sell gold coins via a professional dealer, he or she needs to make money on your transaction to stay in business. Figure a commission somewhere around 10 or 20 percent to go to an honest dealer. Don't be fooled by shysters who pay you half the value, or sell you coins at double value. Such shysters are all over the place, so watch for them.