For just about anything you can think of, the cause of prices going down is technology. Technology made it easier to acquire or produce many of the things that were very scarce and valuable before 1400. Some examples:
- Pepper and other spices. Before 1400, these were brought overland from Asia to Europe and were quite rare. Ship and navigational technology allowed ocean voyages to bring spices in large quantities. The lowered the price of spices. The same process occurred with things like silk.
- Books. These once had to be written by hand. The printing press dropped their price tremendously.
- Clothing. When the whole process of making clothes (spinning thread, weaving into cloth, sewing) had to be done by hand, it was time consuming and laborious. Industrialization made clothes much cheaper.