There is a nice video on Channel 9 about a study done by Microsoft Research about TDD and it’s impact on a real industry scenario.
The study was done post mortem, so the developers were not influenced while developing the product. The results indicate that the Quality of the code is substantial, but there is a slight penalty (on ma hours) due to time spend writing the test.
In any case the defect rates drops between 50 to 90 percent, so subsequent development and maintenance should be cheaper.
You can download the paper on the Empirical Software Engineering Group site make sure you click in the Local Copy link and not the title and you will save $34 .