The topic of slavery is certainly a sensitive one. But it's also an important one. A slave was a substantial investment in antebellum America, and it was an expensive item in relative terms. Purchasing a slave could be seen as a mark of status, the way that today's middle and upper class aspire to purchase expensive cars as a symbol of their economic success.
A typical adult male slave could sell for $1,200 in New Orleans in the decade before the Civil War. A female slave would cost a little less. This price may seem high, but it should be remembered that the purchase of a slave represented an investment in an industry that depended heavily on the cultivation of cotton. Cotton was a crop that demanded enormous amounts of land and labor, and it produced much more revenue than other crops such as sugar or rice.
But it's also important to remember that the purchasing of a slave was not the only way to gain access to its labor services. Slaves were also hired out for a specified period of time, with the majority of slaves being rented out on yearly contracts. The hire market for slaves moved in a pattern that was very similar to the sales market, and even had a slight lag in some cases.
Previous research on slave hire rates has varied widely, but most of it does fit fairly well with Fogel and Engerman's data-series from probate records (with the exception of the late 1840s, where it is not far off). A general rule of thumb was that a master was paying for a slave's full 24 hours of working time each day, including any nonworktime activities.