Well if it helps, I just took AFFOC not too long ago, and there were some people who came into our class just for the MED C part. Not sure if it is cheaper to do so, but you can always go to the school and ask the instructors or the people at the office. Sometimes it is better to go to the instructor, as they usually can help you. For the MED C however, it does require that you go for the firefighting, as I had to do that before I got the certificate. So I don't think there is a way around that now, they have combined it.
For us they had the MED C and D put together, maybe it was just a special occasion but it’s better to check anyways. You don't want to be paying for something you don't need.