An attendee of a presentation I gave told me that she charges too little for writing personal histories but can't make herself ask for more. She asked me, "How do I overcome my fear of asking for what I'm worth?" Here's how I responded:
Conventional wisdom is that many people, disproportionately women, are afraid to ask for what they're worth. I used to believe that, but more recently have concluded that a person's reluctance to charge a higher rate or negotiate for a higher salary may well be a rational appraisal of his or her fair market value, that which the buyer or employer would be willing to pay.
That person may, for example, know that he or she is not that brilliant or hard a worker, or that a person of equal or greater merit could be found at the same or lower cost.
That person may also be considering the likelihood that asking for more money will result in the offer being withdrawn. (Indeed that has happened to my clients and to me--twice, including at CNN.) Especially when, for that person, the real-life value of the extra dollars isn't great, the person may be wise in deciding not to ask for more money--better to have the job for less money than to risk not having it at all.
In sum, don't prematurely dismiss your intuition on how much to ask for. Consider your fair market value and the real-world risk/reward ratio of asking for more, and then decide.