It's hard to say, but the International Federation of Journalists has 600k members, so maybe there exists 6M journalists worlwide, of which maybe 10% are investigative journalists (600k IJs). If they are paid like $50k/year, that's $30B used for IJ.

Surely from browsing the internet and newspapers, it's clear than less than 1% (<60k) of journalists are "investigative". And I bet that half of the impact comes from an identifiable 200-2k of them, such as former Pulitzer Prize winners, Propublica, Bellingcat, and a few other venues.

Let's say that each person affected gains 0.1 to 10 QALYs for these high profile case

This seems very generous to me. The example here that effected the largest number was about tax evasion by Pakistani MPs; even if previously they had paid literally zero tax, and as a result of this they ended up paying full income tax, it seems implausible to me that the average Pakistani citizen would be willing to trade a year of life for this research. I would guess you are over-valuing this by several orders of magnitude. 

IJ in developing countries where they're needed the most probably have a full time job doing something else, and probably don't make more than $4k/year. In first world countries, sure, go for $50k/avg. 
Obviously when you focus on something you can bring down the price, but attacking corruption at such a big scale does bring up the cost. IJ aren't enough. Lawyers are needed, but they're all pro bono because they know this work needs to be done. 
 We're talking billions of people affected. Yes the numbers can be completely different because you just can always say "Well there's still corruption in the government, so if the billions of $ of taxes that were being evaded are now funding corrupt policies & corrupt corporations wasting more resources etc...." It's exponential, and it just can't be attributed due to secondary affects. 

Check out CongoHoldUp.com, there's bloomberg articles about it etc. 
How much corruption witholds funds that could have helped the people in extreme poverty in the world? 

These global health interventions mean squat if you're trying to put a bandaid on an open wound. That's what corruption is, a cancer deep down, but if it's too deep for most here to comprehend, let's just call it an open wound. Mosquitos are flying out of this wound and causing malaria, call it a fever, just a symptom. You keep investing/donating and the problem will never be solved, but you clap yourselves on the back, you're saving more lives for less spent. The wound is still open.
So how does that in the end affect your QALY IIUC?
Saved a life, now the kid needs to survive living in poverty still, to hope he can get a good education and a good job someday, but then spend 30 of his adult years to finally buy a home because the price of housing in Nigeria increases 100% every year. (OK on average... over 9 years it's increased 700% in many areas)

You have France who exploited African countries for decades, here's one example: 

Exposing this is just the first step to real change. How much is real change worth? To close the wound, to stop the cancer. Transparency to end corruption is the only real long-term effective solution. 
 

A case study of Investigative Journalism solving an important problem

I think this bar is too high, given most top charities do not claim to "solve" a problem. Philanthropic dollars going towards reducing malaria burden, reducing number of chickens slaughtered for factory farming, reducing AI X-risk can be impactful even if the problem is not solved.

I think if there were case studies that showed IJ was a good use of philanthropic resources on the margin, or directionally supported the claim that "EA should focus at least 10% of its effort towards both IJ and connecting their work with such organizations that can do sanctions, take organizations or politicians to international criminal courts, and reparations" that would be useful, even if they didn't solve an important problem.