The Big Mac index: Value meal
economist.com
economist.com
http://siteresources.worldbank.org/ICPINT/Resources/icp-fina...
For example, the demand for big macs may be different in different countries, which would lead to price discrepancies. The same thing goes for supply (# of restaurants, etc).
Also, the branding of a Big Mac is different in different countries. A McDonald's in Paris or Brussels is generally more "upscale" than a McDonald's in Chicago. That difference in branding can also impact price.
So, other than for the purposes of amusement, or understanding "Big Mac" valuations, the "Big Mac Index" is not really a valid comparison metric.
http://research.stlouisfed.org/publications/review/03/11/pak... (PDF)
Canada has better value than US, despite having near identical standards, why? My bet would be that every McDonalds employee receives minimum benefits (prescriptions, glasses, dental, IIRC) due to the health care being handled by the government. The interesting thing is that it used to cost more in the UK, and does in Europe still, with similar systems. In fact the UK system is more integrated than Canadian, so following the logic from USA->CA should mean the UK costs less (you don't have to cover children's dental, and there's barely any company benefit plans in the UK, at least in the sectors I worked where ironically you'd need most: construction) however it cost significantly more until this year.
So why in a system with more established care systems that remove the burden from employers did it cost more? This is where judging the prices of globalised products is interesting.
My guess for the UK would be higher taxes for the consumer, worker and business drove up the cost of the burger.