Norse Atlantic seems to be having a sale. The best deal based on cost per mile flown looks like flights from NYC to London.

JFK-LGW - $99 one-way (2.8620 cents per mile / CPM) - https://www.google.com/travel/flight...NXUCvBYzsok9N7
LGW-JFK - $253 one-way (7.3453 CPM) - https://www.google.com/travel/flight...zP7Y56qXA5LUZA

JFK-LGW - $352 roundtrip - https://www.google.com/travel/flight...rE3fyXW4aMNNv8
LGW-JFK - $352 roundtrip - https://www.google.com/travel/flight...8fX2AsyER9tLQ6

MIA-LGW - $157 one-way (3.5360 CPM) - https://www.google.com/travel/flight...9UP3SVasUnk1w7
LAX-LGW - $196 one-way (3.5766 CPM) - https://www.google.com/travel/flight...YRyk54cLmc2Dy7