If per-capita GDP is $137, how can a slave which cost $1,500 offer a 10% ROI?
That slavery continued in the South suggests that it was at least slightly better than the proposition of bringing in immigrant laborers, but it wouldn't have had to have been much better. The biggest disadvantage I could see to immigrant labor would be the difficulty of making a smooth transition, since nobody who had slaves would have wanted to take a major hit on their investment.
Market inertia can be strong; I'm curious to what extent slavery persisted because it was economically superior to immigrant labor, and to what extent it persisted because of inertia.
Largely because of the presence of slavery, few immigrants came to the South. It's not like southern landowners really had a choice which method to use. Nobody wanted to compete with slave labor, a major factor behind northern opposition to slavery and its spread.
There is considerable evidence that plantation slavery, on a well-run large plantation, was more "economically efficient" than free labor. One source I ran across claimed that plantations were about 40% more efficient on average than northern family farms, and about 60% more efficient than southern family farms.
This is, of course, the efficiency for that particular organization. It seems pretty clear that it was not an economically beneficial system for the society as a whole. Just as illegal immigrant labor today can be highly profitable for a restaurant or lawn service, while having detrimental side effects that the whole society pays for.