Or from credit card churning, manufactured spending, or, in the most extreme case, pudding.

I'd like to hear more about pudding.

Reference to https://en.wikipedia.org/wiki/David_Phillips_(entrepreneur)

I presume that was a reference to this fellow: http://gizmodo.com/how-an-engineer-earned-1-25-million-air-m...

International upgrades still tend to require fairly substantial co-pays. Nothing like paying the whole thing in cash but still in the multi-hundreds of dollars.

That depends, I recently booked a "round the world" trip in first class with miles (total distance traveled 19500 miles and 5 different cities visited so not really round the world) and in total I only paid 400 usd in taxes and fees and 205,000 miles.

If you can get the miles from churning, I wouldn't say that the co-pay is particularly onerous, it's much less than it would have cost to buy all the tickets in economy.

That's pretty good. United is around $500 one-way for trans-oceanic upgrades to business plus around 25K miles. A pretty decent bargain over just buying a business class ticket but not a trivial cost either. I do it now and then but I'm selective.

That's impressive. I want to now more, is there any way to reach out to you?

Try http://www.flyertalk.com/forum/

Spending company money means you're rich

I've flown company-paid business class flights that cost something like 5% of my pretax income. Who spends 5% of their income on a weeklong trip multiple times a year?