Japan and the UAE have risen dramatically in this year's Mercer global Cost of Living Survey, prompting experts to warn potential expats to consider carefully.In the study, released earlier this month, Tokyo moved up one spot to become the world's most expensive city for expats, with Osaka taking second position, up from 11th position last year.Meanwhile, Dubai climbed 32 places to reach number 20, principally due to the UAE dirham being fixed to the US dollar.Commenting on the UAE's performance, Nick Marr, chief executive of homesgofast.com, said: "This should be taken into consideration by employers who are sending expats to the region and is all part of the effects of the global economic crisis."Abu Dhabi, the UAE capital, also surged from 65th position to become the 26th most expensive city to live in as an expat.Johannesburg was touted as a top destination for potential property investors as it is now the cheapest major city to live in as a foreigner.