How much is a 1977 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 Proof Krugerrand. However, a good estimate for a 1977 Proof Krugerrand would be about 115% of the gold price.
On the 15 August 2016 a 1977 Proof Krugerrand was sold on ebay for US$ 1474.50 which would be around 110% of the gold price.