Rents vary widely depending on the location you wish to stay in and owners demand a huge deposit. It is standard practice to ask for a deposit of anything between six months to a year upfront. This is before you have even moved to your rental home.
Also remember that until payments are made and documents signed, anything can change. It is common for landowners to promise you an apartment only to give to someone else if the other party offered a higher rent. So make sure everything is in writing and that you move in as soon as possible. If you're planning to rent an apartment in Mumbai, it is best to go through a professional real estate agency. There are a number of websites to help you in your house hunt. Below are a few to get you started.