Final answer:
The cost of slaves changed the least between 1820 and 1830.
Step-by-step explanation:
The cost of slaves changed the least during the ten-year period between 1820 and 1830. After the ban on the international slave trade in 1808, the number of slaves in the South only increased by approximately 750,000 in twenty years. It was not until the cotton boom in the mid-1830s that there was a significant demand for enslaved people, leading to a dramatic increase in prices.