ask them to cover your travel costs at the very least (flight/car/hotel/food). ask them afterwards what they might have paid, or what they might typically pay for a speaker on this subject or of your (limited) fame. Given that you've not done it before, and you've done it for free, you must like the talk/topic, so ease in to this with travel costs covered. They may see fit to give you a small honorarium of a few hundred dollars on top of that.

I recently read Scott Burkund's "Confessions of a Public Speaker", which was awesome. In it, he details much of the finances and logistics of being on the speaking circuit.

Some interesting points that are related to your question:

The elite speakers (current and former presidents (our penultimate president hopefully excluded), CEOs, popular authors, etc. have a set speaking fee. The range is wide, but can be anywhere from 10k to many hundreds of thousands per engagement.

Everybody else gets paid whatever the event has budgeted. You and your talk don't get to have a fee. An event has a budget and they may offer you something better than the next spot, but at the end of the day, they decide.

Lots of speakers are presenting at places that are barely paying for travel and accommodations, in hopes of attracting more influential and higher-paying gigs.

So, your talk sounds great, and has a specific target audience. I'd seek out the best conferences for that audience and try to get in. Once you're there, you'll see what the pay day looks like, but probably not until then.

How much value would you get out of speaking if it were free (assuming they pay for travel)? If you told them a price and they said no, would you be okay walking away from the opportunity?

I left out a lot of details so this post doesn't turn into veiled self-promotion, but I'll happily answer if any of you need more information.

Identify that maximum amount you'd be comfortable charging, then add 15%.