How many people are going to attend it? The prospect doesn't mention that. Or who the organizer is. What's the track record?

Judging by the prospectus it's the first conference you've ever organized and you cannot make any predictions about the turnout (or know it's going to be lackluster).

If that's not the case then you haven't communicated it[0].

Otherwise 500$ as the cheapest entry point is unreasonable. Quite frankly at this stage, as an unknown brand - they're the ones doing you a favor.

I'd lower the prices AND make it clear they can participate without paying a dime. You can charge $$$ next year/iteration.

[0] you mention some luminaries - well I don't know who these people are. a simple byline like "who created <library x> used by <y> people around the world" would go a long way.