One of the common doubts that arises in the minds of most developers is, "How am I supposed to estimate my effort when we adapt to TDD?" As developers are supposed to write unit and integration test cases as part of TDD, it is no wonder you are concerned about how to negotiate with the customer or management for the additional effort required to write test cases in addition to writing code. No worries, you aren't alone; as a freelance software consultant myself, many developers have asked me this question.  

As a developer, you test your code manually; instead, write automated test cases now. The good news is that it is a one-time effort that is guaranteed to help you in the long run. While a developer requires repeated manual effort to test their code, every time they change the code, the already existing automated test cases will help the developer by giving them immediate feedback when they integrate a new piece of code.   

The bottom line is that it requires some additional effort, but in the long run, it helps reduce the effort required.