It's fairly clear (and distasteful) that economically, slave labor is inefficient because you have to feed/house them or you lose the 'investment'. In contrast, you can pay someone well below a living wage with virtually no penalty. This is why so many people at the low end of the labor pool have multiple jobs.

the sad truth is that slave labour needs no startup cost beyond a fist.

Fists ain't cheap.

Slaves are capital, and, like all capital, acquiring and maintaining them has nonzero cost.

In the antebellum American South, some tasks like ditch-digging were sufficiently hazardous that a plantation owner would rather not risk their investment in their slaves, and so would hire Irish ditch-diggers by the day, as the plantation owner would be out less money if they died while digging the ditch.

Somewhat off topic, but I can't figure out how ditch-digging would be particularly hazardous? The only danger I can think of would be a wall collapsing, but that could surely be avoided..

I'm assuming heat related conditions.