It is hard to measure how much they really stole and these type of estimates really don't mean anything. The fact is that no one really knows how things would have turned out had the British never come. Chaos theory says that even a flap of a butterfly's wings can cause a storm on the other side of the world like several months later. Even something as insignificant (on macro terms) like the death of a single individual can totally change world history and here we are trying to imagine what the subcontinent would have looked like without colonization, a totally impossible exercise even with all computer models, simlations and what not.
it is entirely possible that without the British Raj, the region would have been even a bigger hellhole with dozens of states fighting and having GDPs lower than Somalia,
Or its possible that the region would be a world superpower with the highest living standards known to man.
Since theres no way to determine what would have happened, hard to say how much they really stole in practical terms,