Although there has been some written on the subject of economic
comparisons of slave and wage labor, I don't think anyone has arrived at a
definitive answer of whether slavery was cheaper.  If the self-interest of
slaveholders was at all rational (in economic terms), then they certainly
seemed to think it was cheaper.
 
However, from the point of view of British protectionists, the relative
cheapness of slavery did not have to be an established fact, merely a
useful political argument.
 
Tim Dennee
Alexandria Archaeology