Helen Tunnicliff Catterall's 5-Volume "Judicial Cases Concerning American Slavery and the Negro" of which I assume you speak is, as you say, quite expensive at over $400 where it can be found at all.
The reason for this, I believe, is simply that the book is and has been out of print for quite a while. When books go out of print, obviously, no new copies are available to fill the demand. As in all economic scenarios a limited supply of a product will always increase its cost to those who demand it.
You might consider writing the publisher (William S Hein & Co - http://www.wshein.com) and asking them if they have any plans to republish the book in print or perhaps in ebook/kindle format. You should know, though, that as a specialty publisher focusing on legal texts their prices are still well above those you may be expecting from normal mass market books.