To be blunt I think you need to be technically excellent and publicly recognised as such before hanging up your shingle to do this kind of work. Once you have people asking for you to print their negs then you worry about how much to charge and how long it takes.

If your customers are happy to pay what you ask then all good but if they baulk, you can probably then think about how to optimise process and/or whether the time you spend on a neg is reasonable.