Zerodium and similar 0day sweatshops pays a lot for these 0days. Bug bounty programs directly thru the companies have been documented to screw over researchers.
Zero-day brokers pay in tranches for completed reliable exploits; bug bounties pay in lump sums for (generally) POCs. It's not an apples-apples comparison.
Can confirm.
ZDI pays lump sums AFAIU.
ZDI is more like a bug bounty than a grey market broker.