I believe that EasyJet charge depending on how many seats are available; the more seats availablr the lower the price.
In the BBC news item mentioned by malanda, they stated that everyone they spoke to paid a different price, but most of the people they showed paid around £50, which if they are correct at £5500 per trip would need 110 pax, or abour ¾ full. Saying that the flight did apear to be well subscribed, and they are noted to fly only to the most popular destinations.