Transportation from Mumbai Airport: Taxi, Bus, Rail

 

Mumbai Chhatrapati Shivaji International airport is the unique airport that serves Mumbai, but this airport was created by the fusion of two old airports and some people of Mumbai refer to it as two separate airports. The Domestic Terminal or Terminal 1 was Santa Cruz Airport, and the International Terminal or Terminal 2 was Sahar Airport. Both terminals are 5 kilometers apart, so be sure which terminal are you arriving / departing (Some flights to other indian cities departs from International Terminal).
See below the different transport options that you have in Mumbai/Bombay Airport:

Inter terminal Transportation: There is a paid service that connects both terminals (Domestic - Terminal 1B and International - Terminal 2).
T1B to T2: Prices Rs.245 for a Sedan. Rs.700 for a SUV.
T2 to T1B:
Prices Rs.230 for a Sedan. Rs.750 for a SUV.

The counters to buy the ticket are located:
T1B: Arrivals. Phone: 8767350021
T2: Level 2, International Arrivals. Phone: (022) 65882672

Taxi

Get to downtown Mumbai from Mumbai Airport within 25 minutes up to 2 hours.
There are two types of taxis operating from the airport: The normal metered taxis and the prepaid taxis.

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Rickshaws

Auto rickshaw is a metered mode of public transport that just operates in the nearby suburbs and also from Mumbai Airport Terminal 1.

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Bus

There isn't direct service between Mumbai Airport and Mumbai city centre, but there are some routes that connect you with train stations offered by BEST and MSRTC bus companies.
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Train

Mumbai Airport hasn't an own train station inside the airport, but you can use any of the available means of transport (Bus, Rickshaw, Taxi) to reach the nearest stations.

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Car Rental

If you want to have completely independence from public transport and taxis, the best option is to rent a car.

You can compare the different options available at Mumbai airport, at car rental page.

If you are planning a large trip inside india, probably the best option is to use railway instead of renting a car, since the distances between cities are really big.