People looking for rock-bottom airfares find them by using agents called consolidators, who have arrangements with airlines to sell excess capacity at low prices. Not to disparage the product - I spent many years seeking out those kinds of fares - but the airlines with excess capacity are sometimes third-world carriers. Not always, and that's not necessarily bad. You can probably buy a decent rtw from a consolidator for around $2500 in coach class (although it's been a long while since I've looked).

At the other end of the price scale, you should be quick to befriend anyone willing to spend $25k on airfare for an rtw-like trip. They are very likely to be foolish with their abundant fortune, and perhaps some of it might come your way.