Depends on the RTW. At the moment Europe isn't the cheapest place to buy premium cabin (A/D)ONEs. Assuming the USD figures for Romania and Bulgaria are typos, Sri Lanka, Iran, Pakistan and various African countries are cheaper. Oz, New Zealand and the UK remain lowest for LONEx itineraries (which don't get a lot of attention here on FT but I suspect represent the majority of sales of Oneworld Explorer tickets.)

But "cheapest" is such a relative term. How much will it cost you to get to the "cheap" origination point? (Not just airfare or award miles, but also hotel nights, visas, wire transfer or credit card conversion fees, etc.) Is it worth it relative to your itinerary? For example, if you start in Romania you can't fly LHR-DXB-LHR because you're only allowed 2 flights between London and the "longhaul Europe" cities, and Romania is one of the countries on the list. On the other hand, if you start in Frankfurt, you'll pay more for the ticket, but you can travel intercontinentally to Asia, North America, South America, or Oz right off the bat. You can save all four European segments for the end of the trip, rather than "burning" coupons to get to an intercontinental gateway.

Or maybe having a TA or an airline office that knows what they're doing is worth something. Recent experience with BA in some of their smaller stations suggests this might be an important issue.

Plus the new members (with their route structures) might shuffle the deck big time. Japan is currently priced (A/D) comparably to some of the lower-cost European origination points. With JL's enormous route structure, and competition over the Pacific and from Europe to Japan, maybe Tokyo is the next "hot" origin point.

Too many factors to generalize. One person's cheap is anothers PITA.