Jim, in the book The South Was Right, by the Kennedy brothers from here in Louisiana, the subject is basically treated as being a case where slavery was no longer profitable in the indistrialized north and was more expensive than paid labor. The institution was thus abolished but done over an extended period of time to allow the slaves to be sold off in southern markets. I can't recall if they provide percentages but their thought was that while these states abolished slavery they did so to appease the business community, not out of any moral ideals. At least this is how I recall the subject and their conclusions.

My friend, if you haven't read this book I recommend doing so. That being said I found some of their conclusions and claims difficult to reconcile. Still, it is enormous food for thought. I will be glad to loan you my copy if you need it.