Get a meetingburner pro account and charge each person a flat fee. You could say something like "I normally charge $XXX per hour but because I'll be talking to XX number of people I can charge $XX instead."
The pro account integrates with Paypal and you can use the the free trial if you want during the webinar and not pay anything. MeetingBurner - Features
You could try taking the approach of allowing the participants to bid for a slot. Top 20 slots get in and the highest bidder gets to ask the first question. Let the free market decide the value of the person presenting.
You could try taking the approach of allowing the participants to bid for a slot. Top 20 slots get in and the highest bidder gets to ask the first question. Let the free market decide the value of the person presenting.