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

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

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.