My SIL did this. She has a full time job and her husband makes good money but she teaches yoga at the studio where she did her training, 2 or 3 classes a week. I honestly have no idea how the financials work out (e.g., I'm sure the training was expensive and I don't know what she makes at the studio and when she will "break even") but she does enjoy being involved in the studio in that way. I think she enjoyed the very intensive course as well. I would consider what the potential to teach after doing the certification is (I know there were a lot of people who SIL did the course with and I can't imagine they can all get absorbed by surrounding studios) and whether you will feel more fulfilled as an instructor or more enjoy taking classes and learning in that way. That's more my perspective as someone who is largely outside yoga but it's definitely something I would consider before doing a teacher training course.
momma to DD 12/08 & DS 3/13