Hiya!
Welcome to London!
In general -
I have many many relatives and thier friends in and around NYC, Manhattan & L.I and I know how relatively expensive it can be over there. In comparison to London?, well it depends exactly what and where in London you want and are. Manhattan equates to Central London, the heart of the city, all the career people and all the nightlife and glitz of city life. I'd estimate London is around 25-60% more expensive that Manhattan in this respect. The 25% figure is probably related to more mundane items, groceries, clothing and up to 60% for things like Fuel, housing, utilities costs etc. Of course, those figures can vary for individual elelments!
US salaries for the same job are substantially higher, plus there's more of them there. Here if you look through various industry specific publications and those which include job advertisements, you can thumb through and almost see the 'amount' of high paying jobs compared to lower paying ones. There's not 'that' many in the higher bracket!
As an example, I'm in the IT industry and worked for 2 large US IT companies (made redundant from both!) and theres a weekly IT publication that has a job section. It has little league tables (almost) and an interesting one is '%age of adverts in IT with salaries from 0-20K - 20K-30K 30-50K and over 50K, highest percentage was in the 20-30 K range and a very small amount in the over 50K section!
Industries which seem to buck this trend seem to be anyone invloved in Law, Medicine and Finance (from my observations - theres probably other areas as well of course) - but then when you compare those to the equivalent pay in the US prepare fo your jaw to hit the ground.
London really can be alot more expensive that Manhattan, but again, depends where you are - like for like, then yes be prepared to be shocked!
The US/Sterling rate is a little more attractive for USA people right now, but like mimsy says, the effect may be negated when it comes to how much is in yer paycheque each month and having to live in London!
Do yer sums first is basically the first step !
Cheers! Dennis! West London & Slough UK!