If it was so cheap to hire workers . . .
. . then why did production of Carolina Rice end with the end of slavery?
Besides, once you got a few slaves they were self replicating, and the surplus could be sold off (you say for the amount of 20 white men's labor for a year for each one).
And, of course, there was the "Fancy Trade", selling young black women as sex slaves, at a nice price. Hard to give that up, no?
And, if slaves were so expensive, exactly how could they all be bought and released? Maybe Salvation Army bell ringers could raise the money?
In any case, the southern slave owners would see that was a very short term pay off, that would be as severe a long term destruction of their economy as emancipation was.
And besides, the South was raring for a fight. They expected the war to be very short, and to be victorious over the "Servent Race".
I question your economics.