When I went I worked on the principle of $100 a day for food/travel/entrance fees (eg Empire State), though it will depend on how much shopping you want to do! You can use the ATMs if you need to, though obviously there are charges for this.
Yeah above answer sounds about right am going for 10 days and taking double that! don't intend to go shopping mad but its all the tourist attractions i want to see! will soon eat up the cash and eating out!
check out virtualtourist.com and tripadvisor.co.uk for tips and stuff on what to see and do in new york
enjoy
:)