Your one mistake was that your agreement was not properly made.
Your price is not at all off the mark.
As an experienced EC, the time involved in the job could be a lot more than 15 hours. But because of your pride, professionalism, and skill level you were able to do this in the 15 hour time period. I believe that is worth more than most give us credit for - as she could have had a person with less skill who may have done a horrible job.
Chalk this up to experience, eat what you have to and try not to do it again
Pierre