If you are travelling alone, or with a family not more than 3, it is cheaper to use KLIA Ekspres, which is a non-stop airport rail link service. If you are more than 4, I advise you to use taxis or some other apps like Grab or Uber. Average one-way taxi fare is around 120-100 MR (Malaysian Ringit) (approximately 18 GBP, 23 USD).
KL Sentral, which is the transportation hub of Kuala Lumpur, is the final destination of KLIA Ekspres.
KLIA Ekspres has two stops in the airport for each terminal namely KLIA and KLIA2. KLIA is the main terminal. KLIA2 is the terminal mostly for low-cost airlines and the newest one.
Journey takes 28 minutes (KLIA – KL Sentral), 33 minutes (KLIA2 – KL Sentral).