In the May/June 2007 issue of IEEE Software magazine there was an awesome series of articles about Test Driven Development and I've just discovered that one of them - the main one - is now freely available: Guest Editors' Introduction: TDD--The Art of Fearless Programming by Ron Jeffries and Grigori Melnik.

Go get it, read it all and spend some time studying Table 1 on page 28:

In particular the last two columns: Productivity effect and Quality effect