Spices were one of the most desired commodities in the 15th and 16th centuries, along with gold and other precious items. Their importance as flavor enhancer already existed before then. Salt and cinnamon were also used as food preservatives. However, that does not explain why their price was so high.
The reason is mainly consumer-demand.
At the time, spices were seen as something exotic from far away lands, lending an image of wealth, social status and prestige to their buyers. They were seen as "luxury goods". Much like designer brands today cost a lot more than regular brands simply because of their name, spices were famous and, therefore, expensive. The belief that spices had medical properties also contributed to their fame.
