This is a very good article.
Work for free or for full price. Never for cheap.
If you charge too much though they will walk away. You might even feel guilty charging the rate you want for doing the thing that you love, but very few people are in the position that you are in. You bust your ass for a reason and not everyone can do the excellent work that you do so be proud of it and charge a rate you can be proud of. Most companies have no problem paying for quality work and yet most designers have a problem charging for it.
If you do a job for free you can control the expectations of the client. You can tell them what the boundaries are and what they should expect of you. When they aren't paying anything they don't want to overstep those boundaries to piss you off. They are thankful for any work that they can get out of you.