From my own experience, a 1 hour webinar ends up taking up about 10 hours of your time in preparation. They work best when there is a set topic, and questions are answered only at the end.
As far as payment, you could announce it and take donations to give out scholarships to those who can't afford it, but really need it. There was that lady online at Christmas who offered 5 gift cards for those who could prove they needed it for Christmas, and while she was giving them away, others on the thread were offering to pick up the next person after hers were gone. They ended up giving out over $14,000 in gift cards!