TDD takes longer. Studies conducted at Microsoft and HP (or possibly IBM), found that development time with TDD was 10-30% longer. That's probably minimum, and in high degree determined by the TDD experience level of the team members.

The massive upside is that there is much less defects in the end product (think it was 50-80% in the study, but can't recall exactly).

Depending on the project, the extra time needed, might or might not be worth it.

Take a somewhat complex web app, that is used for a marketing campaign. It has a life time of only 2 months. In this case it does not make sense to use TDD, as the lower defect rate and "future-proofing" qualities, might very well not be worth an increased budget.