There are a couple of good articles on Spearmarketing regarding Webinar data.
My experience is:
Webinars: 40-50% attend rate, usually between 10a-2p EDT
Roadshows: 50% attend rate if free, breakfasts work best in most cities. 75% attend rate if small fee and good content. More general prediction content worked better for me, but that was probably industry specific.
Registration rates totally depend on Content+Target+Timing. If your topic is timely and deals with predictions about a topic people care abotu (but not too niche), then you should get registrations. If your list is properly targeted, you will get more registrations. I know of a company that gets 350+ per webinar in a niche space because they are focused on their list.
Subject Line: Topic + Date (but not too long)
CTA: Register Now/Today
Drive traffic from syndicated content sites, newsletter ads, and house list.