Hey, even a Bob Ross painting will cost you something
I agree with @Rich Green and @DanetteDraws, I typically don't do work for free. I value my skills as an artist, and I value myself as a professional, whether or not I've gotten paid work. If someone likes your art but says they aren't willing to compensate you, I think that speaks a lot more about how they value your time and skills.
There are a lot of other articles, blog posts, and books written out there by people who can better articulate why working for free is so damaging to both your career and the illustration industry as a whole. (I can hunt down some links for anyone interested in that stuff)
And really, why work on someone else's passion project when you can work on your own? Personal projects can really help build your skills, your portfolio, and possibly get you noticed.
Speaking from experience, I did some freelance work for a low wage (making less than MW in the end). It was stressful and not really worth the pay, and I did feel put out and devalued quite often. I've also had people ask if they could use previously finished images for various projects, and I've always turned them down. Again, I won't work for OR give away my work for free.
I do make some exceptions. I sometimes do gift art for family and friends, but mainly it's of my own desire and not because I was asked. If they did come to me for free work I would most likely turn them down if it wasn't something I really wanted to do. Also, I'd be willing to do something "for free" if it was a project or collaboration I was really interested in. So, along the lines of doing a piece for a comic anthology, working with an artist/writer that I really like, or a show that really peeks my fancy. But again, it would have to be something I was passionate about and not just because someone asked.
Sorry this post was so long, but it's one of those things I feel strongly about.