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.