If you are just flying for the miles then I agree with landspeed - a RTW trip will run about $3,500 + taxes for a 3 continent trip.

If you want to take the time to see a bit of the world, however, then the RTW can be a great value. The problem is timing - it's not worth it to fly into a city, only to fly out that night or the ext morning.

The ideal RTW trip is one where you can get a map of the world and start sticking pins on places you really want to see and build the trip up from there.