When is Going "The Extra Mile" Too Much?

Recently, I met with a prospective client. She asked me for my credentials and I verbally went over them and later sent her some links to sample writing. This was not enough to convince her to do business with me. So she asked me to write a sample for her based on information about her product.

I am trying to build my business and do not have very many clients, therefore I obliged. She sent me some links and some ideas she wanted in the writing and I sent her my sample. It took me about an hour to write the sample; every word has to be just right. 

Starting a business is tricky in this regard. Everyone tells you that you have to build a solid reputation and follow through, but what about working for free? Potentially, I will get this job and then I will be paid to complete the rest of the work, but as of now, I'm not visiting the bank. 

What things have you done to gain or keep a client?

Taking a break after working for free