The wonderfully named Expatistan.com website has a tool showing the comparative cost of living in various cities around the world.
It shows Beijing as being 61% cheaper to live in than London, which to me sounds somewhat on the high side – I would have guessed it as nearer 70-80% cheaper as long as you don’t insist on buying Western brands all the time. Imported goods have a huge tax rate on them (despite mostly being made in China in the first place) so the use of a pair of Adidas trainers or an Italian meal as a comparative cost is skewing the results somewhat.
Have a look at your city: http://www.expatistan.com/cost-of-living