Is a slave zero cost though? Seems like the opposite. You have to pay a large upfront cost for whatever extra marginal output they can produce, which is minim

Is a slave zero cost though? Seems like the opposite. You have to pay a large upfront cost for whatever extra marginal output they can produce, which is minimal because you still need to feed/house the slave and their family, but they're probably not the most motivated worker. So you have an expensive worker you need to feed with low productivity. The economics of slaves seem pretty poor quite frankly.