KLIA Transit, sorry. From KL Sentral buy a ticket to Salak Tinggi and get off. Cross the walkway and buy a ticket for KLIA.
Wait maybe 30 minutes and continue your journey. I did this once, but now just take the 10% discount on the full fare by buying at a kiosk.
Are you saying that there's a discount on the KL Express now? If so, it didn't occurred me that there was one. When I arrived at KUL, the rep was explaining the fare to me and told me the kiosk price is the same.