Here's a link to the current rates: http://en.wikipedia.org/wiki/Air_Passenger_Duty
The rates are based on distance from London to capital of the other country; for U.S., that's 3661 miles and the rate converts to about $108, twice that for any class above cattle class.
The APD particularly bites for Caribbean people living in UK; they're in the next band up and that means a family of 4 visiting home pay about $750 round trip on top of the tickets!