British occupation of Egypt ended in 1922. This silver 50 piastres is dated 1956 and commemorates that event, signified by the broken chain. The coin contains 0.810 troy ounces of silver and it can never be worth less than that.
To evaluate your coin, first compute the base value (BV) by multiplying the current price of silver by 0.810. Look up the price of silver at kitco.com. It changes every day. If, for instance, silver is selling at $15 US dollars per troy ounce, the BV of the 50 piastre coin is 0.810 x 15 = $12.
Coins in excellent condition sell for substantially more than BV, as follows:
average circulated: BV + $8
well preserved: BV + $15
fully uncirculated: BV + $40