How much is a 1999 Proof Krugerrand worth?

As the value of bullion Krugerrand coins is very much tied to the price of gold it is difficult to determine the price of a 1999 Proof Krugerrand. However, a good estimate for a 1999 Proof Krugerrand would be about 110% of the gold price.
On the 18 June 2016 a 1999 Proof Krugerrand was sold on ebay for US$ 1799.99 which would be around 135% of the gold price.
On the 4 July 2016 a 1999 Ultra Cameo PF68 Proof Krugerrand was sold on ebay for US$ 1985 which would be around 147% of the gold price.