Largest ransom ever paid was in 1532 when the Spanish conquistador Francisco Pizarro captured the Inca Emperor Atahualpa. Atahualpa was held for eight months during which Pizarro extracted the largest ransom in return for a promise to free the Inca emperor. A promise he did not keep as he executed the emperor after being paid the ransom, in a ridiculous trial. The ransom was enough gold & silver to fill a room 22 feet long by 17 feet wide to a height of over 8 feet. It is difficult to specify how much they paid. However, it is recognized as the highest ransom paid in the history of mankind.
Atahualpa was awarded the last two options: be baptized as a Christian and then hanged or be burned alive. Atahualpa Chose the first option. He was baptized with the Christian name of Francisco, executed on July 26, 1533.