It's worth zero millions dollars to curl. It might be worth something to attackers, but "wide use" is not the same as "the ability to compromise everyone" because most of the time, curl isn't used to automatically fetch untrusted URLs in a way that's controlled by the attacker from an environment they care about. It's worth much, much less than a 0-day in an iPhone or PHP. You'd need a buyer with a very specific itch to scratch and there won't be a bidding war.

By extension, if a vuln is worth something to attackers, it's probably also worth some fraction of it to their potential victims. But if you're an interesting target, your defense can't be based on the ability to find out about every single software defect ahead of the time. You need layers. And if you have that, the importance of proactively acquiring every 0-day is diminished.