These old gold pieces from France are valuable. Yours has some damage, so its value will be limited to the value of the gold it contains. If it were in good shape, your coin would probably be worth $100 US dollars more than the gold content alone.
Here's how to calculate the value of the gold content. First, look up the value per troy ounce on a web site such as kitco.com. Then multiply that value by the amount of gold in your coin. In the case of a Louis Philippe 20 franc coin, that is 0.1867 troy ounces. Note that when speaking of precious metal, the word 'troy' is understood, so 'troy ounce' and 'ounce' mean the same thing.
For instance, at the time of this writing gold is running $1105 US dollars per ounce. Tomorrow it will be different, so be sure to look it up. The base value of your coin is then 0.1867 x 1105 = $206. A worn or damaged coin will be worth that much. A coin in excellent condition will be worth $100 more (or even more), which, in our example is $306.
Be sure to check for better dates and mint marks in the series. If you have one of these, the values go way up: