Absolutely: Panama Canal. And in terms of level of intervention, I think it's fair to say that the U.S. has almost never interfered/intervened with the affairs of another country quite so thoroughly. Bold claim? Sure. But the facts back it up.
First, consider that the U.S. potentially ginned up the Panama City revolution, and at a bare minimum, U.S. military support on November 3, 1903 was critical to the survival of the revolution. The U.S. consolidated those gains three weeks later with the Hay–Bunau-Varilla Treaty, which gave the U.S. precisely what the Columbians had previously refused to grant: permission to own, operate, and profit from a completed canal. So there's the military investment: through the hard work of the USS Nashville and the Marines on board, the U.S. won with military pressure something they hadn't been able to accomplish diplomatically.
Next came the financial investment, which obviously was substantial. The all-in cost for the U.S. was almost $400M in contemporary dollars (i.e., not adjusted for inflation). I've never been able to find a good figure for total tolls paid, but in some senses that's irrelevant when deciding whether the investment "paid off." Through two World Wars and a Cold War, the U.S. controlled the Canal. Our ships, both military and civilian, and the ships of our allies, had full use of the canal, and our enemies' ships did not. The commercial and military advantages were amazing. And the investment paid substantial knock-on benefits: the Gulf Coast port towns in the U.S. saw a substantial increase in shipping tonnage due to the Canal, and those ports took economic opportunities away from developing ports in South and Central American which could have served as coaling/refueling stations if ships had continued to round the Cape.
An absolutely awesome source for all things Canal related is The Path Between the Seas by David McCullough. In addition to being a fascinating topic (or so I think), the book is also extremely well-written.
What do you mean by "financial investment"? Are we talking the US government helping prop up a foreign government's spending, funding development projects, or what?
Also, what counts as an "intervention" for your purposes?
The korean war and subsequent support for the south korean regime have at a relatively low cost provided the US with a very strong military ally in east asia and a major trade partner
Richard miller estimates the cost of the korean war as 678 billion 2007 dollars
While according to the US trade representarive bilateral trade with South Korea was 113 billion dollars in 2014 alone
When you factor in the strategic and military benefits its pretty clear korea was a good investment
I would also add Greece shortly after WW2 because using the Marshall Plan, the United States quite literally bought out emerging Communism. Therefore, this could be constituted as a financial win.