The reason why the prices quoted above are so expensive is because of the rules which the lower fare tiers have attached to them.
The majority of outbound Monday, return Friday flights will be that expensive as, correct me if I'm wrong, but I'm fairly sure the cheaper fare tiers require either a certain number of nights or a certain night stay (e.g. Saturday).
Looking for example at departing Monday 1st and returning Sunday 7th:
BA - £624
UA - £692
KL/AF - £484
EI - £603
In my own personal experience, I went to NY in Feb on a Monday-Friday package. Flight only would have been around 1600pp yet the package with the same flights was around 700pp so it just depends when you are travelling and in what way as to whether the cheaper fares are available.