My favorite rooftops bars/cafe's in South Mumbai are located in 5 stars as mentioned by
@Baboo and hence are definitely not pocket friendly. If you are curious, I am referring to Aer at Four Seasons in Worli, Asilo at the St Regis in Lower Parel and the Dome at the Intercontinental near Churchgate.
In addition to the spots recommended by
@Baboo in Colabo, I would encourage you to check out the Bayview Cafe at Hotel Harbour View and the Marina Upper Deck at the Sea Palace Hotel. Both of these are located next to each other in Colaba, pretty close to the Radio Club and are actually cheaper than the Rooftop at Yogi Executive... that is according to Zomato's cost estimator for 2 people.
Enjoy!