Actually, to veer from the subject of this thread for a bit, slavers only needed about 25% of their cargo to survive to pay for the trip and turn a profit. This gave birth to a controversial theory; that black Americans excelled at sports because only the physically strongest of them survived to pass on their genes.
Yes, but they still had a strong incentive to keep more alive.
Paying expenses and making a 15% profit is nice, but not nearly as nice as a 200% or 500% profit! IOW, once the costs had been sunk, every additional slave that survived to be sold was pure profit.
Now, there’s an incentive!