The low-cost model is that prices are low to start with, until bookings come in, then they start to rise as the aircraft fills up (as I'm sure LTNman knows).
From one website re-the SEN flights: "departing southend to alicante thu 5 april return 12 april lowest fare £124.98!! compare that to £86.98 from stanstead." (I haven't checked them myself though). I'm sure there are still some cheaper ones around though.
And, yes, I'm sure the novelty value coupled with the initial marketing has greatly helped these SEN sales, but I'm sure that the attractive product being offered by SEN also helps.