If anyone doubts this, just take a look at what Khan Academy currently offers for a fresh perspective. Make an account and click around (https://www.khanacademy.org/signup); it should be easy to see that a ton of effort went into making this and, as far as I can tell, they run purely on donations. I wouldn't be surprised if the current cost of operating Khanmigo is just being passed along by Khan Academy at no markup, or even a slight loss.
For parents who want to use it with their children, $4/month (for your entire family!) is very small compared to anything out there that you want to use to supplement your kids education (other than Khan Academy itself, which is free). Is it affordable to poor families in Bangladesh? No, but the barrier to sponsorship is low: "for $50K a year you could sponsor it for 1000 families in Bangladesh" is not a difficult sell.
Also, I expect Sal to raise funding to give it for free to those who can't afford it (just as he did with Khan Academy). At least it's not some VC-backed venture that we know is going to be look for a 10~100x return, so we can be thankful for that.
https://www.khanmigo.ai/pricing
Yes, teachers can use it with students if they pay for it. One day it might be free for students, but it presently is not. Which is what I was pointing out to the person I originally replied to. This discussion is about Khanmigo, a subset of what Khan Academy provides, and it is not free for students (though, yes, that cost is not exorbitant). But it does make it difficult to claim that Khan Academy is free when the specific feature being discussed is not.