The fact you have even suggested prices as low as this tells me you should get yourself on a short business course, or at the very least spend a solid year researching the portrait business before trying to enter it. Do some sessions for friends and family for free. When you become good enough to charge... then charge properly and with confidence that you are not undervaluing yourself and/or undermining the market for professional portraitists. I say this in the nicest, most constructive possible way.
On the rolls of film/number of shots front... you cannot charge like that. What if you slip focus on one, or blow the exposure on the next? If you monetize each frame, then each frame the client will want to see - so you loose the ability to edit or experiment.
Film costs and processing are minor compared to labour and other business expenses. Charge more than enough to cover them. Also, you can only sell what you actually shoot... 15 frames gives you NO flexibility to sell prints.