Good on you for doing all that research TongueTied.
When checking out Kobo's ebook prices I compare between the 3 - Aus, Canadian and US and buy the cheapest.
I haven't done a strict survey as to which country is consistently the cheaper, but I think it's the US, then Aus, then Canada. (And bear in mind, the AUD/US exchange rate is in our favour.)
I gain the most when a title is not available in Aus, but it is in either the US or Canada.
I miss Books on Board because my account reflected Japan with them - and wow - the ebook availability was even greater. (I paid in USD).
Sony US can be a good buy as well.
My two cents ....