Slaves weren't cheap, but the income and wealth inequality of those times was incredibly larger than today. In many jurisdictions slaves actually were just property, not people. Wealthy slaves owners still could kill slaves just for fun, without feeling much financial loss. Like today someone could buy a Tesla and then decide to burn it.

(Though we should recall that the rights of slaves and whether slave-owner could be prosecuted for manslaughter obviously depended on era and jurisdiction).

I find it astonishing that people compare modern-day income equality and the position of the poor in our time, in the Western world, to that of historical slavery (slavery in United States, or slavery in other slave-based economies in the New World prior to 19th century, not to mention slavery in the Old World which continued officially well into later half of 20th century).

In the West, no one is legally killing, whipping or dismembering the poor workers for not being obedient. Even the poor are still free to leave a place if they find life there unsustainable. Modern-day comparisons to "slavery" typically involve something along the lines "you are expected to do something in exchange for money that is given to you and you feel unpleasant about it".

It just doesn't compare with the actual, terrible injustice of real slavery.