A few thousand bucks buys you a professional conference speaker. Most of that is expenses.

I had a friend who did conferences for verticals that would attract 500-2000 people. People who write books, former generals/admirals/nasa people, B-list celebrities, people associated with past news events, tech celebrities, etc all do keynotes.

A remote event should be much cheaper as these people have no work. I’d offer $500 and go as high as $2000.