TDD ROI Metrics

Case studies were conducted with three development teams at Microsoft and one at IBM that have adopted TDD. The results of the case studies indicate that the pre-release defect density of the four products decreased between 40% and 90% relative to similar projects that did not use the TDD practice. Subjectively, the teams experienced a 15–35% increase in initial development time after adopting TDD.

http://www.springerlink.com/content/q91566748q234325/?p=7fd98b01480f49e2925f36393c999a72&pi=3

http://blog.agiledad.com/2009/07/cost-of-bug.html