What is the value of a 1975 500 Balboa gold coin?

The value of the coin is a piece of information which is very difficult to find. The gold weight is 41.7 grams (900/1000 - 90% gold purity). The value of the gold content is over $2000 AUD - the coin is very rare (especially in mint condition) and as yet I have been unable to find the mintage number - does anyone know how many were struck?

I would estimate the value of this coin to be approximately $4000 AUD or more depending on condition.