What is a 1986 gold half dollar coin worth?

The U.S. does not make gold half dollars. For one thing they'd be worth about $700 at today's prices and not even the most boneheaded bureaucrat would make a coin that lost the gov't 699.50 on each one.

Simply put, you have an ordinary cupro-nickel half dollar that was plated for use in jewelry or similar. To a collector it's a damaged coin worth 50 cents only.