Slaves were expensive. As the years of slavery passed, they became more expensive. Regardless, white farmers, plantation owners, and others bought slaves as an investment. Slaves made even more money for their owners than they cost to buy.
A healthy 18-year-old slave could be bought for $650 in 1845. (About $14,500 in today's dollars.)
That same slave could be sold five years later for $1,000. (About $21,000 in today's dollars.)
That same slave, if he or she were still healthy, could be sold again. In the years right before the Civil War, such a slave might bring nearly $2,000. (About $40,000 in today's dollars.)