Current cost is $45. Many writers swear by Scrivener.
Description from Wikipedia
Scrivener is a word-processing program designed for writers. Scrivener provides a management system for documents, notes and metadata. This allows the user to keep track of notes, concepts, research and whole documents for reference (documents including text, images, PDF, audio, video, web pages, etc.). After writing a piece of text the user may export it to a full-fledged word processor for formatting.
Features include a corkboard, an outliner, iPhoto-like full-screen mode and "snapshots" (the ability to save a copy of a particular document prior to any drastic changes). Because of its breadth of interfaces and features, it has positioned itself not only as a word processor, but as a literary "project management tool", and includes many user-interface features that resemble Apple's software-development environment Xcode.
Keith Blount created the program as a tool to help him write the "big novel", allowing him to keep track of all of his ideas and research. It is his first application, built mostly on libraries and features of Mac OS X from v10.4 onward. In 2011 a Windows version of the software was released. It is written and maintained by Lee Powell. A Linux version of Scrivener remains available in beta form.