More than I had thought. And Lincoln really was the difference maker:
…prior to 1860, few political events seemed to affect slave prices, and even the Dred Scott decision had only a small and temporary effect. After Lincoln’s nomination for the presidency, slave prices fell, and they continued to fall once the war commenced. The overall decline in slave prices was large (more than one-third from their 1860 peak) and occurred prior to any battle losses by the South.