How Cathedrals were Afforded and Used
Up until about 1000, people in Medieval Europe wouldn't even think of building a large cathedral. There were very few towns and cities with people to build and fill the cathedral, which was one of its purposes. Another problem was money. Most small communities barely had enough money to feed themselves, so their simply wasn't enough money in one community for a cathedral. However, at the start of the 1100s, town began coming together and the power of the Church began picking up steam. Trade routes which had been unused for hundreds of years suddenly saw traffic improve rapidly, and people had more money to give to the Church. This was one way that chapters raised money for a cathedral. But there are also many other ways,
The chapter of the local church, made up of monks, was the group in chanrge of raising funds for the church. Most of the money for a cathedral was from donations to the parish. Many of the large stained-glass windows in cathedrals were paid for by large donations from a wealthy person or group. At Chartres Cathedral, pictures of the doners at work can be found in the windows, like a butcher selling meat or a cobbler making shoes (Chartres Cathedral, 37). An interesting thing about the donor windows is that the merchants and guilds got the best spots, down low near the entrance. The local lords were often put way up high, demonstrating the growing power of merchants (Lage, 43).

Donor Window (Miller, 16)
Churches, with the approval of the town council, could also tax the populus if they were raising money for a religious cause, like a new altar or a cathedral. These taxes, combined with donations to the parish, made a lot of money, but often they weren't quite enough. To raise extra money, the chapters would display relics at the local church and charge people for the benifit of viewing the skull of a saint, or a piece of wood from the cross of Jesus. When the local people had been bled dry, the chapters took their relics on tour to other towns and cities. However, there are records of relics being on tour in two places at once, and the Papacy voted to end the showing of relics without permission from Rome in 1215 (Lace, 43).
When the showing of relics was banned, chapters found another way to make money: have people pay the Church to forgive them of their sins, although the payment was technically called a donation. This way of getting donations grew into the most profitable way of collecting money for the Church, and was on of the key factors that led to the Prodestant Reformation (Lage, 43).
The most important part of a cathedral's job was to be the seat of the bishop, but the also funtioned as regular churches. Mass was held every Sunday for the people in the parish, and special celebrations such as St. George's Day and Easter. It was also a town meeting place, and if the cathedral had relics from a saint in it, it would most likely had a big festival on the day of the saint. There were probably more uses for these magnificent buildings, but they are now lost, along with the people who built them.