Nice find, the verdigris has come through after 200 or so years.
Even though its an old coin, even in nice condition its only 20 or so pounds. In that shape maybe 2 to 5 quid, but as a conversation piece, its priceless.
I think too, its the beginning of your new hobby!
Loving Halfcrowns. British and Commonwealth coins 1750 - 1950 and anything Kiwi.
If it's round, shiny and silvery I will love it.