Stansted has actually received a lot of BAA investment, one way and another. The whole terminal complex is only 20 years old, and there have been many investments in it since then. Plus lots of additional roads, new hangars and aprons all add up. That is why BAA want £1bn, and that's the amount the new owners will need to service from operating STN.
It's one of Ryanair's complaints that the regulatory asset base (on which charges are levied to make a return) is both too high, and rising.