Having worked in both india and the US for McKinsey I can say that the salary in india is significantly lower for BAs and Associates especially. India also has a 'Junior Associate' position usually for grads fresh from IIM and they make much less than associates in the US.
India is cheaper than the US but once you take out rental costs it really isn't that much cheaper. You get different comforts such as help at home but that's about it. The salary gaps is extremely large and in my opinion not justified