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.
Not everything is priced on demand and supply -- at least not strictly.
Of course the potential of abuse is part of the equation, but I think Google (or similar large companies) simply has a guideline of how the amount of the bounty is decided, than surveying the market to see what its "actual value" is. It's not exactly a free market, at least not on Google's side.