Let's assume that Hernando Pizarro was telling the truth about the size of the ransom and that all the promised ransom was received. Here is the description from Hernando Pizarro
for that he could give them ten thousand plates, and that he could fill the room in which he was up to a white line, which was the height of a man and a half from the floor. The room was seventeen or eighteen feet wide and thirty-five feet long.
We'll start with the room. Assuming that the average height of a man was 5 feet, that would make the volume of the room to the line 17ftx35ftx7.5ft. 4462.5 cubic feet. One cubic foot of gold weights 1206 lbs so the total amount in the room would have been, accounting for open space, less than 5,381,775 lbs of gold. There is approximately 280,000,000 lbs of gold above ground in the world. So that would make it about 1.9%.
Here is a picture of silver Incan plates, let's assume the gold ones Pizarro describes are the same size. They are not very big, so let's put their weight at 2 pounds. That would add 20,000 lbs of gold to the ransom which even at 10 lbs a plate is a negligible amount compared to the world's gold supply.
According to my copy of 1491, the room Atahualpa filled was 22 feet by 17 feet, and it was filled to a height of 8 feet.
According to the BBC, the total amount of gold in the world is about 67^3 feet^3.
If you assume that the room was filled by a solid block of gold, it works out to be a little less than 1% of the current gold supply. Of course, there was probably much less gold than that - many objects, like cups, are mostly empty space, and the objects wouldn't have been perfectly packed in there. I think you can reasonably say that the amount of gold Atahualpa was ransomed for was somewhere between .0001% and 1% of the current world supply, and is probably closer to the range of .01% to .001%.
