Vulnerabilities will always sell for more on the black market because there’s an added cost for asking people to do immoral and likely illegal things. Comparing the two is meaningless.
To give a straightforward answer: no, I don’t think $20k is underpaid. The severity of a bug isn't based on how it could theoretically affect people but on how it actually does. There's no evidence this is even in the wild, and based on the description, it seems complicated to exploit for attacks.
No, it's priced on demand and supply like anything else; bug bounties are priced to be the amount that Google thinks it takes to incentivise hunters to sell it to them, vs. to black hats.
He got nothing. No money at all. The CEO pretended to have forgotten every verbal agreement.
You only need to experience that kind of thing once to change your mind.