What is value of a 1979 Queen Elizabeth II Hong Kong gold 50 cent coin?
Nobody could afford to make gold 50 cent coins, your coin is
nickel-brass.
A Hong Kong 1979 Fifty cent coin, uncirculated and in absolute
mint condition could fetch up to $1 USD. If it has been circulated
but still in good condition, it might fetch anything from $0.20 to
$0.50 USD.
A reputable coin dealer will be able to give a more accurate
valuation.