“They are just going to be working the day of the shoot…”
Wrong.
They have been working at what they do for years, perhaps decades, to be capable and available for what you want to shoot. And that’s what you’re paying for. Can’t minimize the importance of that. You’re not just paying for the guy to talk; you are in a sense paying for everything he knows and all the work he did to learn it. You really can’t afford to pay THAT amount, of course, so what you ARE paying is a symbolic recognition of his expertise. And reimbursement for his inconvenience.
My own experience tells me that you don’t want to get into a residuals or points situation with everybody you tape, where you’ll have to constantly get back to them every month to tell them how many units sold and how much from that is their cut… all on your word. Not that your word isn’t good, but why should anybody have to take it.
No. Go with a flat fee and be done with it, in this case, based on what they charge for a day of instruction, plus a little bonus. Either you can afford to pay them a reasonable fee for what they will do, or you can’t. In the case of “can’t”, perhaps they have an altruistic reason for appearing, like a doctor with a message about people’s health, who might forego a fee because it’s part of his oath to help people this way.
Rest assured, if that same doctor is asked to be an expert witness in a courtroom, he ain’t doing that for free, even if it’s the same exact information he gave away publicly.
At least go for an honorarium and plenty of screen credit, but establish that it’s a one-time amount up front. Always remembering that without the guy, you HAVE no show.