Land was cheap before the conquest, so devout donors could earn themselves lots of heavenly brownie points by giving ( or most likely bequeathing) land and other valuables to religious houses. The pre-conquest houses were all much bigger and richer than the post-conquest foundations. The choir monks and choir nuns could not earn money, as they spent their time in singing and praying. They needed the income from the farms ( etc) to live on. Sometimes a rich person entering a religious house would more or less buy his or her way in. A widow, for instance, could retire to a convent and take her property ( her husband's bequest ) with her to donate to the House. Houses were not supposed to charge an "entrance fee" but they all did. Choir monks and choir nuns were recruited from the top levels of society, and lay brothers and lay sisters were from lower classes. These did the spade work, cooking, cleaning, probably. In fact, the abbeys owned so much land that there was a favourite mediaeval joke which went "if the Abbess of Shaftesbury could marry the Abbot of Glastonbury, thier heir would own more land than the King of England". It would have been true, too.