The expression “costar un ojo de la cara“, in English “to cost an arm and a leg”, is used when something is really difficult to do, in terms of work and sacrifice.
Its origin is more literal than we could think. In fact, the inventor of this expression really had to pay an eje. (NB. “costar un ojo de la cara” literally means “to cost an eje in the face”).
The protagonist is the explorer Diego de Almagro, who, during an exploration between 1524 and 1525, was attached and wounded with an arrow by an indigenous, who hit him right in the eye, leaving him one-eyed.
Afterwards, Diego de Almagro told to the monarch Carl I: “Protecting the interests of the Crown costed me an eje in the face”.
Owing to the risks experienced by Diego, this sentence was soon used to underline and emphasize the diffuculty of an action, till changing, over the years, into the current meaning.