London Stansted Airport Can Anyone Give Me Information On The Cheapest Way To Get From London Stansted Airport To City Center?

Can anyone give me information on the cheapest way to get from London Stansted airport to city center? - london stansted airport

Come 5th November in Brussels to London Stansted airport to downtown. We arrive at 925am and return the same day at 840pm Stansted airport in Brussels. Can someone tell us to do the cheapest way for us, too? It is the dream of my sister and we are there for a day. Thanks for the help!