Supply and demand! The prices will rise until the supply will reach the demand.
From what i remember back in like 7th grade slaves were treated like items on a ship, they had little to no space and were crammed up together, i remember being shown a picture drawing of them even being put in the storage area. There, they would not be able to move much nor leave. all their wastes were not cleaned resulting in sickness being spread. Many were also hungry. At the end of the journey, lots of slaves died due to the reasons stated above.