Why is The Turner Diaries novel by Andrew MacDonald so expensive?

I’m taking a sociology class this semester titled “Radical Social Movements” and one of the novels we have to read is Andrew MacDonald’s “The Turner Diaries”. This book is written by a white supremacist and former leader of the National Alliance. It’s full of racist propaganda and is extremely anti-Semitic (I’m sure I’ll be on the FBI’s list after purchasing my copy). While shopping around online, I noticed that the novel is selling in some markets for practically $250! Another seller on amazon is selling a copy for $500. So here’s the ultimate question: WHY?