You can choose between a multiple bus companies to travel to or from Stansted airport and London. Sometimes you can get a cheaper ticket when you book it few days or weeks before your journey. It is often possible to get cheaper tickets when you buy also a return ticket.
Check these bus companies:
National Express - you can travel to or from multiple stations (Victoria station, Liverpool Street station, Statford station, Golders Green station etc.) or bus stops. Buses departue quite often, sometimes even every 30 minutes.
Airport Bus Express