It was clearly not about economics to southerners, and most everyone with prospects in life lived under the belief (realistic or delusional), that they could one day save-up enough money to buy a slave, the way one might afford a house or a car.
Economically, southerners were willing to take on two massive opportunity costs - secession from America, and then a war with the industrialized opponent. Clearly, economics did not play a role at all when it came to the southern defense of slavery as an institution.