I came across an excellent summary by Steve McConnell of the benefits of software process improvement, Business Case for Better Software Practices.
A key differentiator, and justification for looking at the article, is Steve's inclusion of examples and numbers. Interestingly, Steve also uses this article to introduce the concept of "rightshifting." Bob Marshall is a good source if you are interested in following the Rightshifting thread or you can view a video introduction to Rightshifting by Bob and Grant Rule.
Finally, you can look at our own approach to Rightshifting effectiveness. Please share if you know of any other good Rightshifting resources!