How much does it cost to go from London Luton Airport to central London? Is it possible to go by metro? And if not it's possible to buy the ticket right there or it has to be booked online?
You can get to central London by:
The airport has its own railway station "Luton Airport Parkway", and is served by trains 24 hours a day from Central London using "First Capital Connect Trains" and connects with St Pancras International. There are up to 10 trains an hour, depending on the time of day. All trains go to London St Pancras International, but many also continue on to Blackfriars, London Bridge and Elephant & Castle, Gatwick Airport and Brighton. The station is nearly 2km (1 mi) from the terminal building, there is a shuttle bus service running between the terminal and airport every 10 minutes, costing £1.50 each way. At rush hour times, this journey can take up to 25 minutes.
This costs £13.50 one way.
National Express or Greenline run from the airport. £14, or from £1 if you get the ticket ticket booked in advance online.
When I used to go, I'd get the rail option generally, but if you can get a good deal with National Express, or even EasyBus transfers which aren't mentioned on Wikitravel, you can get tickets for £2 from them if you're lucky (although usually it's higher).