There are also several busses to the city, and even to other airports. There is no cheap option however, as public transport in UK is expensive and Stansted is still quite a distance from the city. The ´fast´ bus still takes about an hour to get there. I think I paid somewhere like 14 pound for a single busticket 2 years ago.
And don´t look to the fare only, London is big, so if you know where you want to go in London, it makes sense to find the bus/train connection that gets you to close to it.
You can find all the options on the easyJet website by the way:
http://www.easyjet.com/en/airport/London-Stansted" onclick="window.open(this.href);return false;
Erwin