One reason i refrain from asking for money for software is that the creators of almost all software i use in my life and 100% of the software i use to create software don't ask for money.
As my father once told me, "Son, anything worth getting is worth asking for."
Share your talent freely with those who need it, strike a fair bargain with those who can afford it, and sleep comfortable in having made the world a better place.
Or, in other words, pure altruism isn't sustainable.
The beauty - and paradox - of open source is that people produce software, in an act of self-interest, for free.
Ask for money when it makes sense. Give it away when it doesn't.