I’m not sure your analysis on the reasons for the end of the Atlantic Slave Trade (which is not identical tot he abolition of slavery in general) is correct or anything approaching correct. There were numerous reasons for the abolition of the Atlantic Slave Trade. Philip Dwyer, Professor of History at Newcastle University writes this while refuting Steven Pinker’s book in an academic analysis The Better Angels of our Nature;
A recent analysis of various cases of abolition around the world reveals the extent to which other motives, often masked as humanitarianism, played a role. The British naval campaign against slavery from the mid-nineteenth century was motivated by a desire to enforce abolition, yes, but was also driven by the British desire to control the seas, as well as a personal desire on the part of many navy captains for prize money.
Dwyer, P. (2018). Whitewashing History. Historical Reflections/Réflexions Historiques, 44(1), 59.
Of course, as Dwyer notes, Enlightenment values also did play some assisting role. So it doesn’t appear to have anything to do with “low costs” at all. Secondly, you’re simply wrong about this being an abolition of anything other than the Atlantic Slave Trade. By the time the Middle Ages ended, slavery had almost been wiped off of the European continent. This is because Christians came to believe, towards the end of the classical period, that the New Testament ruled out owning fellow Christians as slaves – only pagans. As the number of pagans continued to dwindle as the masses voluntarily became convinced of the message of Christ, the potential pool of slaves vanished into near existence, and did indeed almost end entirely by the end of the Middle Ages.
The Atlantic Slave Trade erupted in the century after the end of the Middle Ages which brought back slavery into the forefronts. But in any case, some sort of “abolition” (of which took place over a very large extent of time) did take place during the Middle Ages due to Christian motives.