I get about £130 extra if upgrading a LHR-JFK rtn in WTP to WTP out and CW back, For an exUK booking I am not sure why you are seeing figures in USD.

WTP rtn
Air Passenger Duty - United Kingdom £185.00
Passenger Service Charge - United Kingdom £56.06
Passenger Civil Aviation Security Service Fee - USA £4.30
International Transportation Tax - USA £15.00
International Transportation Tax - USA £15.00
Animal & Plant Health User Fee (Aphis) - USA £3.00
Immigration User Fee - USA £5.30
Customs User Fee - USA £4.60
Passenger Facility Charge - £3.40
Carrier imposed charge £240.00
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Total taxes/fees/charges £531.66

WTP out CW back
Air Passenger Duty - United Kingdom £185.00
Passenger Service Charge - United Kingdom £56.06
Passenger Civil Aviation Security Service Fee - USA £4.30
International Transportation Tax - USA £15.00
International Transportation Tax - USA £15.00
Animal & Plant Health User Fee (Aphis) - USA £3.00
Immigration User Fee - USA £5.30
Customs User Fee - USA £4.60
Passenger Facility Charge - £3.40
Carrier imposed charge £370.00
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Total taxes/fees/charges £661.66