The cheapest way, (one person la) is to take the Air Asia bus to LCCT lor... RM9 only... from KL Sentral also got, one utama also got.

And from LCCT you can take a RM2 bus to KLIA... but tedious i reckon..

if u care about carbon footprint, try the KLIA express lor RM38 from KL Sentral.

Cabs will be like RM80? from KL/PJ.