It's worth it once the reward is at least twice the cost of a full tank of gas or at least 25% of my rebuy cost, whichever is higher.What's the players' definition of "worth it"? 1 billion per hour? 10 billions?
Back when I first started playing and missions only paid 1,000 credits I didn't feel like they were even worth doing in a Sidewinder. And back then, I mostly didn't do missions because of that. I made money by driving my SRV around and picking up platinum and other crap that had been abandoned in some random crater on any random moon.
I'm glad that missions pay more now, because I really enjoy doing them, but I think there's still some work that needs to be done because they often times still aren't worth doing when you're flying a ship with a 12m credit rebuy and they're offering to pay you 500,000 credits to track down and kill somebody who they admit is badass enough to be called a "Pirate Lord."