What was the social structure of the slave based societies?

Let's talk about old slave-based societies where a significant portion of the population were slaves and slave ownership was common. Consider ancient Rome, ancient Greece, the Americas before the civil war and the Caribbean before the end of slave trade. <br><br>In modern societies a significant portion of the population is employed in jobs that are important to produce the necessities to keep us alive and a smaller, wealthier class controls the capital and gets most of the benefits of the labor. It's possible to draw parallels between modern wage labor and historic slave owning societies but there is a key difference: slaves were property, not employees. Slave owners in slave based societies were responsible for feeding, housing and keeping their slaves alive. They didn't have the option of simply firing an underperforming slave and replacing them with a better one because they were responsible for keeping those slaves alive for as long as they lived.<br><br> Slave owners had access to free labor but the cost of owning slaves was much higher than paying someone a wage. This must have meant that slave owners needed to own a lot of slaves to make a profit. This does not seem to be true. From my understanding, in ancient Rome, Greece and in the Americas, owning a few slaves was something a lot of citizens could do and it did not require anyone to be particularly wealthy. This seems contradictory and requires an explanation of how societies structured themselves around a labor system that offers free labor in exchange for a higher cost. <br><br>So what is the difference between historical slave owning societies and modern societies? In other words, how did these societies function differently than modern societies by being able to afford the costs of owning slaves when the alternative of paying a wage is cheaper?<br><br>&#x200B;