I can answer a few questions. Yes there can be very large price differences based on where you purchase your ticket. And I mean a HUGE difference.
Here's some prices from North America (off SQ's site):
http://www.singaporeair.com/saa/en_U...otions/rtw.jsp
I think the SQ/NZ/VS RTW is a very reasonable deal, even though it's ex-US.
Also, NZ flies RTW as well. No price though.
I think ex-BKK has some of the cheapest RTW out there.