Idra brang a lot of money from his stream alone, he was averaging 6k or something recently (having gone down since Demuslim got ok and overtook him), that alone probably brang more (not necessarily directly from Twitch revenue, I don't know who gets that, just from showing sponsors to all the viewers after every game and stuff) into EG than most of the Koreans did (other than Jae Dong they're generally around 500ish viewers). You can't really argue that Twitch viewer numbers aren't important since all EG players have to stream a minimum amount of hours, so obviously EG (because of their sponsors) care about this.
Proleague doesn't get a massive amount of viewers, and even the people that watch that would have to be split between all the players to work out how much each was worth.
When it comes to going to events most of the EG Koreans do badly (apart from maybe DreamHack where the competition is generally poor other than a few) so don't get much/any air time for sponsors (and I never notice Koreans mentioning their sponsors in interviews and stuff like the foreign players do).
Then there's the other stuff he's on like the weekly show they do (Inside the Game?), casting occasionally etc.
He might not bring in the most money, but he certainly brings in a decent amount.
Winning things doesn't necessarily mean you'll get more money from sponsors than bad players who have a "personality" because of how bad a portion of the community are. Idra is/was basically an entertainer like Destiny rather than an actual competitor (although he obviously didn't want to admit this to himself until recently, the same as it took Incontrol ages to admit he couldn't compete), and as far as casual viewers watching player streams and stuff on Twitch go, they'd rather watch entertainers who'll rage/talk and joke with the stream/explain what they're doing than top players just grinding out games on ladder with no interaction/talking etc.
Edit: 4.3k viewers average actually, the nearest Korean was JD with 1.6k average, Stephano averaged 7k and even Incontrol averaged more than the Koreans with 2.3k.