Missionaries cost 100 Gold to train, the same as a Monk. Given its characteristics, the Missionary generally outperforms the Monk in most ways and so is the more desirable choice, especially during the mid-to end game where most of the Relics have been collected.
As Europeans gradually came to understand that Columbus had opened the door to two complete continents, they awoke to the desirability of converting the potentially immense native population to Christianity. The Spanish, Portuguese, and other European nations sent monks called missionaries into the New World to bring about conversion and salvation. To be a missionary required great faith, a strong constitution, and great zeal. They often were required to plunge into the wilderness amongst peoples of radically different cultures and beliefs. Many missionaries suffered despair, torture, and death at the hands of natives who resisted the message being brought to them or identified the monks with the disease and conquest that accompanied them.