Onur Cobanli explains:
Never design for free, because most people measure value incorrectly by cost of things; i.e. the more expensive, the more valuable; this idiotic approach of people to value unfortunately makes your valuable design work worthless; since it is free, it costs zero, i.e. worth nothing. Same for pro-bono work unfortunately.. never design for free, if you wish make a huge discount, but do not do it free if you respect your time as a designer.