At this time (late 2000s, early 2010s) you could purchase open jaw fares (say ORD-NRT-CGK/CGK-NRT-STL) and between the open jaw/double open jaw and sometimes a fuel dump, get an economy ticket for less than $400. I would generate typically 40k-50k+ miles at a time, meaning I was getting essentially a free economy round trip between US and Europe for every flight.

So basically I was pocketing a minimum of a few hundred bucks in profit per flight and visiting quite a few SE Asian locations, even if it was 8/10/14 hours at a time. I’d work on the flights and I saved up enough miles to have visited over 100 countries by the time I finished university at 23.

EDIT: This is still possible to do but much more difficult now that most US carriers only give mileage based earnings for their partner flights. The golden days of this arbitrage are long gone.