What is the cheapest way to get from International Airport to the Mumbai CST? Which are the other options (not so cheap, but I suppose more comfortable)?
The cheapest (that's not walking or hitchhiking) is to take an autorickshaw to Andheri train station (50 - 100) rupees, followed by a train ride to Churchgate (7 rupees), from which you can walk or take a taxi (40 rupees?) to CST. However, this is quite cumbersome and very time consuming. Expect this to take two hours total.
The prepaid taxi booth at the airport will charge you no more than 550 rupees to get you to CST. That's, what, 7 euros? The convenience is more than worth it.