Most ordinary 50p coins found in circulation are worth their face value: 50 pence. Some can sell for more when they are scarce, in especially good condition, or in a sought-after collector edition; The Royal Mint says proof, precious-metal and limited commemorative versions generally command higher prices than standard circulating coins.
For a coin in your change, rarity and collector demand matter more than the denomination alone, so compare the exact design and mintage rather than assuming every commemorative 50p is valuable.