The bus is the cheapest alternative.
As I recall, its first (only?) stop is KL Sentral. Its a large bus bay under the shopping center within walking distance of several 5-star hotels (Westin?). I took a taxi from there to the Sheraton.
In bound, I took the bus and discovered its used mainly by locals i.e. proof its the best deal. On my rtn, I took the KLIA Express, which was included in my multi day transit tix!