One of the initial motivations for the Europeans in sending exploration voyages was to find cheap spices. To be worth such expensive and risky mission, spices must have been worth a lot and must have had very high demand in Europe. How much, in today terms, were they worth? And what is the origin of such extraordinary demand? When and why did the price eventually drop?
Here is my main source for the following answers. EDIT 6/4/2015: I have expanded this answer to elaborate on a number of things.
Around the year 1500, a quintal of pepper in Lisbon was worth up to 38 ducats. A ducat was 3.5g of gold and a quintal was only 60 grams of pepper... So, pepper was worth a bit more then twice its weight in gold! Other spices like cloves, cinnamon or nutmeg were probably worth significantly more than that.
Spices were popular then for the same reasons they are today. But obviously they were more of a luxury item at that time. They weren't expensive so much because of the demand, but primarily because of the supply. It was very limited, and very far away, in a period where transportation was expensive.
A big part of this expense was related not just the technical limits of sailing ships and navigation, but also to protection costs (security against piracy and the like). And before the Industrial Revolution there was not much demand for European exports in the East. That made all Asian imports, including spices, expensive.
In the 1400s, the Venetians and Genoese controlled much of the spice trade in Europe. They in turn had to rely on other intermediaries, which increased the cost. The Portugese started to establish trading posts in Asia around 1500. Then came the Dutch and the British in the 1600s. The increasing access of European traders to the sources of the spices was a key factor that helped to bring down the price from that point onwards, by increasing competition and eventually reducing protection costs.
Meanwhile a number of things increased the purchasing power of European traders. That included 1) huge imports of silver from the Americas in the 1500s 2) the Industrial Revolution, which allowed Europe to produce cheaper textiles and other goods and 3) colonization of India and other places, which gave British and other traders access to new trade items, most notably opium. All of those things would have helped to make spices and other Asian imports much cheaper in relative terms.