How much is a 50 dollar 99.99 percent gold coin worth?

It is worth whatever the gold price is. I'm assuming you have a US 1 oz gold eagle. If you do indeed have a one ounce gold eagle it is worth whatever 1 troy ounce of gold is worth. At the time of writing your coin would be worth about $1350. However, the price of gold fluctuates on a minute by minute basis.