EasyBus, the low cost bus company which provides coach services to and from London and its main airports, currently runs a great sale. You can book your airport transfer for only £1 to London city centre from Luton, Stansted and Gatwick Airport.
This is the cheapest way to get to the city center. Buses are running 24 hours very often. The travel time is approximately 60 minutes but it depends on the traffic.
For more info and booking, please visit the website of EasyBus.