These nice old gold coins were issued by Belguim between 1867 and 1882. They contain 0.1867 troy ounces of gold and, as such, are worth at least that amount. To find the base value of your coin, multiply the current price of gold, found on web sites such a kitco.com by 0.1867. At this writing, the price of gold is $1243 US dollars per troy ounce (it changes daily, so look it up). The base value is 0.1867 x 1243 = $232 US dollars.
The base value, BV, is often called the melt value of the coin. It is the value of the puddle of gold you would get if you took a blowtorch to the coin. With such old beauties like this, there is additional value over melt due to numismatic (coin collector) attractiveness.
For an average circulated coin like the one in our picture, add about $35 over melt to get an approximate catalog value. For a fully uncirculated coin with lost of luster and pzazz, the added value climbs towards $100. Coins with problems like stains, holes, scratches, and cleanings are worth melt value. Summarizing:
worn: BV + $0 US dollars approximate catalog value
average circulated: BV + $35
well preserved: BV + $60
fully uncirculated: BV + $100
If you are a serious collector, you know about the Position A and Position B nuances of Belgian coins. Place your coin on a flat surface, so you can read the tiny lettering that appears on the edge of the coin. When the edge lettering is 'right side up' so you can read it, check which side of the coin is on top:
POSITION A: Shield side on top when edge lettering is upright
POSITION B: Portrait side on top when edge lettering is upright
For the 20 franc coin, this makes a difference only for coins dated 1876. All other dates and positions have values outlined above. But, if you have 1876 position b:
20 FRANCS DATED 1876 WITH POSITION B
worn: $400 US dollars approximate catalog value
average circulated: $450
well preserved: $600
fully uncirculated: $900