I was talking after acquisition costs and the cost of having paid labor. There HAD to be an economic benefit to it else why own slaves at all? Just go out and hire free people.
Free people don’t want to pick cotton.
From a labor productivity standpoint, they were not as valuable as free labor... but. Unlike free labor, they had very high market value -- could be bought, sold or used as collateral and they reproduced at a high rate. You couldn't sell the children of free labor, but you could sell slave children.
On your questions of whether slave labor was more or less expensive than “free” labor — you can be 100 % certain that the relative values of slave versus free labor was fully reflected in the free market prices of slaves.
Throughout its history here slave prices rose steadily and were never higher than in 1860.
This strongly suggests that over time slaves did increasingly valuable work at economical costs, even with increasing slave prices.
Indeed, one certain clue that slavery was becoming “obsolete” would have been steeply falling prices for slaves — but that never happened.
In fact, second only to the value of their lands, slaves were the South’s largest investment, a fact which more than adequately explains their uncompromising response to any suggestions that slavery was wrong and should be restricted or abolished.