For OWE (the One World Explorer round the world tickets) I have just discovered this site, which includes a spreadsheet which will show you the prices in any currency you chose for OWE tickets issued in every country where there is a price - the differences are dramatic!
http://members.shaw.ca/fewmiles/oneworld/
(You have to get the currency conversion from another website - there are full instructions in the spreadsheet.)
Gibraltar & Slovakia look like good places to buy OWEs in Europe, South Africa is the cheapest for many types.