I think there should be a reduced fee for the first two, but really, no more than that. Birth services should not necessarily be given away unless there is financial need to request that.
Your time and your skills - no matter how green you are - are valuable. In fact, many times a new doula will offer more energy and enthusiasm than a seasoned one. That is worth a price.
I also believe that when people pay for services they are more likely to take it seriously and respect the relationship.
That is not to say that services cannot be reduced or volunteered for those that need it.