It is unusual for a modern coin found in circulation to contain actual silver. But this one does. It contains 0.671 troy ounces of the precious metal.
To find the value of this coin, first compute the Base Value (BV) by multiplying the current price of silver by 0.671. Tomorrow it will be different, but today's silver price is $15.70 US dollars per troy ounce. So today's BV is 0.671 x 15.70 = $10.50. Be sure to look up the current price of silver.
Next compute the face value in your currency. According to xe.com, one Belgian franc is worth 0.0274 US dollars, so 250 francs is $6.85. Even though the Belgian franc has been replaced by the Euro, it is still prudent to factor in the face value if you have many coins. It will not mean much to one coin, but if you have 100 of them there is some residual value that becomes significant.
Taking the BV and a percentage of the face value (FV), a worn and undesirable coin is still worth, say BV + 0.5xFV = about $14 USD. Using that as a starting point, catalog values run like this as condition improves:
worn: BV + 0.5xFV
average circulated: BV + 0.5xFV + $2
well preserved: BV + 0.5xFV + $10
fully uncirculated: BV + 0.5xFV + $15