Best way to go from T1 to T2 in Mumbai

I just had to perform this exact transfer in January of this year. Unfortunately, there is no longer a free shuttle between the two terminals. (We asked the people at the information desk, they told us they stopped operating the shuttle because of "terrorism" concerns. I suspect it's actually due to cost, since people needing to transfer between the terminals are those booked on separate tickets, since the only airlines where someone can book a connection on the same ticket now operate directly out of Terminal 2. Thus, ridership on the shuttle would be much lower, so it didn't make economic sense to operate it.)

The only way to get from Terminal 1 to Terminal 2 is by private taxicab. Claim your bags at Terminal 1, then step out of the terminal. Once you're out, look for the "inter-terminal transfer" counter in the row of counters on the right side. It should be all the way to the right.

We were traveling as a group of four, with eight pieces of checked luggage and four carry-on suitcases. So, we paid for a van, which cost us 1,000 rupees. (If you're traveling in a smaller group, with fewer bags, there is also a sedan, but I don't know how much that costs.)

In addition to what we paid at the counter, we also had to pay 130 rupees twice directly to the driver, which was handed over to the toll booths for cars exiting each terminal. The total cost of the service was, therefore, 1,260 rupees. Do keep in mind that we were a family traveling with a lot of suitcases.

The ride to Terminal 2 took about 20 minutes, and we had to drive in Mumbai traffic. The two terminals are part of the same airport, but are accessed through different access roads from the highway.

I did the transfer from domestic to International terminal recently. There is no free shuttle. I came out from the domestic terminal and went to a taxi booking counter which was shown to me by the security person standing outside the gate. I paid Rs.280 for a non-AC car with 2 pieces of hand luggage and given a printed receipt containing the car number. No other payments were required. I was shown to the car by the security. I also found there are other options like autorickshaw and ola/uber but this was more convenient and safe at night.

According to https://www.mumbaiairport.com/terminals.php

The 2 terminals are in fact two separate airports 5km one from other. If you have a flight connection landing in T2 and departing from T1, take in mind that the transfer process will take 2-3 hours. Domestic Terminal is busiest during the day while International Terminal is busiest during night Is not allowed to check in more than 4 hours prior your flight.

Both Terminals share the same runways and air facilities but are 5km far one from other, and the only option to transfer between terminals is using taxi or the fixed rate Inter-Terminal transportation service (See Transportation page). https://www.mumbaiairport.com/transportation.php