I have looked for a way to education companies on why training is the best assets that their company would ever pay for.  I have believed for a long time that when it came to doing a software implementation the company that vested more time and effort into training the staff was the smoother project.  Here is one of the best resources I have found that has provided a detailed report of different opinions on the ROI of Training

Conner, Marcia L. "How do I measure return on investment (ROI) for my learning program?" Training & Learning FAQs. Learnativity.com. April 5, 2002.