Spiffy coin, Sam. It is a Jamaica $20 gold piece commemorating the 10 anniversary of independence. The coin contains 0.2532 troy ounces of gold, and that gives it value.
To compute the value of your coin, look up the current value of gold at kitco.com and multiply by 0.2532. Today's price of gold is $1238 US dollars per troy ounce, but tomorrow it will be different, so you must look it up. At today's price the value of your coin is 0.2532 x 1238 = $313. If your coin is in beautiful condition, like the one in our picture, you can add a few more dollars (maybe $30 or so) to its value.
If you were selling this coin to a dealer, he or she would offer less than gold value. A reasonable commission would be 10 to 30 percent for a single-coin transaction. They types of mark-ups are necessary to keep the dealership solvent.
If you were selling this coin to a collector, he or she might be willing to pay a few percentage points more than gold value.