What did the Spanish hope to gain from Columbus's voyage?

After the Ottomans took control of the Mediterranean and overland passages to Asia, the cost of traveling these traditional routes became prohibitively high for the Europeans, both from tolls and from piracy. However, the Europeans did not cease to hunger for spices and other goods from the "Orient," especially India. Therefore, the race was on to find alternative routes to the Far East.
Isabella and Ferdinand, monarchs in Spain, could only look on with fear and envy as wealth poured into rival Portugal after it discovered a sea route around the tip of Africa that got them to India. When Columbus insisted he could get to India from the opposite direction, sailing west, the monarchs jumped at the opportunity to have their own trade route and financed his voyage. Their main goal—what they hoped to gain—was wealth, which would translate into power and prestige.

Spain was hoping to enhance its wealth from the voyage of Christopher Columbus.  The crown wanted to chart a western route to India and the lucrative spice and luxury goods trade.  The Portuguese had already charted a route around the southern part of Africa and held a monopoly in that direction.  Columbus convinced Spain that a western route could be charted.  Queen Isabella agreed to fund an expedition in the hopes that Spain could map out a new path to the eastern markets.  Once the Spanish realized that the lands Columbus sailed to were "unclaimed" territories, they wished to secure colonies that would enrich them with resources and markets.  The Spanish also wished to convert the native peoples to Christianity which would grow the power of the Roman Catholic Church.