By the way Patrick, thanks for starting this very interesting thread.
A few other things to consider.
It is far easier to offer a reduction, based on someone's limited ability to pay, than it is to try to increase your price for those who can afford it.
So it is better to go into this with a higher price, and consider things like student discounts or one-off reductions where necessary.
If you see someone who you really want to try photographing, you can always offer to do a free portrait in return for using the result in your portfolio.
If you are really confident, or really unsure, you could offer to do a few portraits on a "pay me what you feel the portrait is worth" basis. You might learn a very interesting lesson.