There are a lot of factors that account for this. This answer covers the bulk purchase aspect of the deal. Basically EasyJet and the like see cost benefits by only flying a singly type of plane and thus only need mechanics and pilots trained on that air-frame and spare parts are interchangeable etc.
They also see discounts, as mentioned in the comments by not offering things like meals, free luggage etc.
Since they make short haul hops between nations often they may be able to work out deals for hauling cargo as well as people to undercut the cost of your ticket.
I cant find the article but it has been noted that in many cases by buying 100 planes at a time significantly reduces the cost of each air frame. They then in turn sell them in a shorter period of time and don't fly them to the end of their life. In many cases an air frame thats only a few years old will hold good value considering the sever discount it was original purchased at.
You can find some info on the cost of operation for a long haul flight here. Things like fuel consumption rates etc and the like will carry over to short haul flights.