In this example, Davidson and Lytle want us to understand why it would have been appealing for a planter to buy slaves. They invite us to think in the same way a planter might. To do so, they compare a servant and a slave. An indentured servant would have provided cheap labor for the planter. Moreover, he would have required the same things a servant would, such as clothes, food and housing. However, after a certain period of time, the servant would have been able to leave the planter. On the other hand, a slave would have required the same initial "investment," but would have never been able to leave his job. Therefore, from the point of view of the planter, a slave would have made more economic sense.