How much to charge for a lead?

I want to generate Leads in a niche, where is high demand for leads, but I can't find any payout information for these leads. Only thing I found is a payout by an affiliate network, which pays $15 for an active call, which could result in a $20,000 treatment - I think this is far too little for a form submit plus call.

Are there any formulas one can apply for what you should get, when providing form submits, or calls to companies?

You first have to establish a trust flow between the buyer and you.
The common thing I do is to give for free some leads, if they are high quality, they will reach back to you, of course. Then you can set the price.
You can charge for leads (less) or for client. For the second one I suggest you using some kind of service to record the calls (via redirection). You must ask for permission before doing this.

How often do you think someone will buy something for $20k off a phone-call? Exactly. That's why you get so "little" money per lead. I think 15$/lead is great tbh