Can you spell out the economics for us?

Let's assume I am a white-hat hacker with access to this allegedly widespread exploit. Obviously, I understand why a nation-state attacker wouldn't want to share their exploit here but what do I as a random citizen stand to lose? It's not like I care about hoarding the exploit to pwn innocent civilians.

You seem to imply there's some economic reason such a white-hat hacker wouldn't take the $500 but don't actually say what it is.

You, the white-hat hacker would be concerned that your attack exploits payload will be logged either by the potentially malicious site operator who could be using this to gather various working exploits, or by other bad actors who already exploited the site before you. Unless your exploit itself is already well known and fully public that is.