He deserves a higher payout, but 1) telling him to be unethical rather than reporting it responsibly and accepting a not-insignificant reward is stupid, and 2) there's almost no chance this would've garnered more than $5,000 on any black market. This is not something that would grant a cybercriminal access to sensitive or profitable information, unless Facebook is wrong in their assessment of how segmented the server is... and even then, much more work would be required for the attacker (more than an average vuln buyer is probably capable of).