Did plantation owners pay a lot of money for slaves?

Some did. The slavers bought slaves in Africa and other areas, and brought them to the South to sell at a profit. Some were valued for their physical strength, and some were bought to provide offspring: children born to slaves became the property of their owners as well. However, children were expensive and time-consuming to care for until they were old enough to work on their own, so not all slaves were allowed to have children.