I was looking for a way to explain to students why the institution of slavery turned into such a major fight, and the easiest way to do so is to put the price of slaves into a modern context. The value of a prime field hand (a man in his early 20s) in 1850 was about $800. Gold was $20 per ounce at the time, so today, that would be equivalent to about 50x that, or $40,000 in current money. A prime field hand was therefore about as valuable an economic asset as a new BMW 335i sedan. There's a nice graph showing how the value of a male slave rose and then fell with age in 1850. An average slave might be worth $400 in 1850, or the equivalent of a new Chevrolet Impala today.
The large slave owners might have hundreds of slaves--and the work that they did is what made a large slave owner wealthy. Without those slaves, even a planter's land was worth far less, because free labor would not have been anywhere near as cheap.