One of the first lessons I learned while working for an agency was... carefully manage people's expectations. If you do something for free for a client once, twice and then three times - the client has every reason to ask "What? Why?" if you try and charge them the fourth time you do the same thing.
Review all the news about Google buying their neighbour GreenBorder.
People are now expecting free software.
Imagine what might happen if Google try and charge for GreenBorder. Imagine the outcry if they increase the price!