Disproportionally more if you divide it on the user base to get the cost of targeting 1 user when you want them all (and most of evildoers want that exactly).
No, that's not at all how the market for high-end zero-day vulnerabilities work. It's interesting to see people just make random stuff up from first principles. Actual market participants have talked through this stuff; you can just find out empirically.
[1] https://opzero.ru/en/prices/
[2] https://www.crowdfense.com/exploit-acquisition-program/