The cost i have mentioned is what it will take from Hyderabad. The cost includes flight charges(which were pre booked before two months so the rates are not inflated), if you prefer bus/train you may get a difference of thousand from the total cost.
I stayed at Koparkhairne in mumbai, upon reaching i got fresh up and started exploring the city. The first stop is Juhu beach (dont save it for the evening if you are roaming for one day, save the evening for marine drive). In mumbai its better to use Mumbai local trains.Use apps to book local train tickets so that you can avoid queues.