Here is an example from the Cathay site of a May vacation flying from Heathrow,
Itinerary Details
Date Flight Operated By Departure Arrival Stop Duration Aircraft Booking Cabin Rules
03 May 2015 CX238 Cathay Pacific LHR 17:00 HKG 11:55 +1 0 16:45 77W Economy (Q)
04 May 2015 CX919 Cathay Pacific HKG 14:35 MNL 16:45 0 333 Economy (Q)
06 Jun 2015 KA5912 Cathay Pacific MNL 08:35 HKG 10:45 0 16:35 333 Economy (Q)
06 Jun 2015 CX239 Cathay Pacific HKG 12:20 LHR 18:10 0 77W Economy (Q)
Fare Details
Passenger Fare Carrier Surcharges Taxes/Fees/Charges Total fare
Adult x 1 GBP 327.00 GBP 72.80 ? GBP 121.16 ? GBP 520.96
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Total fare
GBP 520.96
In my opinion anything around the 500 quid mark is a seriously good price.