Stansted would not have to be as cheap as Southend in order for it to be affected. It's all about value, not the actual amount the airport charges.
Despite Gatwick being no cheaper than Stansted, many airlines have still moved because Gatwick is basically better value for money as it has more worldwide connections, more globally recognised, better rail access to London etc.
If MAG can add value to Stansted, which is exactly what they aim to do in essence, whether it be reducing costs, improving passenger experience, agreeing deals with more retail brands, offering attractive deals to airlines etc. then Stansted increases it's value, making it more attractive to airlines.
This is potentially dangerous for Southend in the sense that they will have an emerging competitior, adding value, ultimately growing in size which is already handling 20x as many passengers anyway which also overlaps into virtually the whole of Southend's catchment area. The people of East London and South Essex, not to mention the whole of East Anglia and the Midlands, are all included in Stansted's catchment area. Even for start-up carriers, the value is vital.
Last edited by FRatSTN; 9th Feb 2013 at 10:47.