India, or should we say the sub-continent as a whole is said to be at its peak during the reign of Shah Jahan, Mughal emperor c.1600s. The GDP back then has been estimated to be around $74,250 million (1990 US Dollars) [8] While the world GDP was $331,562 million (1990 US$) And the population was around a 100 million (give or take a few million) [9] So the share of world GDP for the subcontinent was around 22.4%. GDP per-capita was around $742.5 per-capita (US$1990). By 1870 the GDP of the subcontinent had actually grown to over $134,882 million (US$1990) while population was around 190 million. The GDP per-capita in this case was almost $710. And the share of the GDP of the sub-continent vis-a-vis the world was 12.05%! By 1913, the population was 257.06 million and the GDP was $204,242 million (US$1990). The GDP per-capita was $794.5 per-capita. The share in world GDP was 13.38%!
The numbers are tabulated below for better comprehension

:-
1600s Indian subcontinent
GDP: $74,250 million
Pop.: 100million (approx)
GDP per-capita: $742.5
World GDP share- 22.4%
1870s Indian subcontinent
GDP: $134,882 million
Pop.:190 million
GDP per-capita: $710
World GDP share- 12.05%
1913 Indian subcontinent
GDP: $204,242 million
Pop.: 257.06 million
GDP per-capita: $794.5
World GDP share- 13.38%
To put it into perspective, our current stats are:
India+Pakistan+Sri Lanka+Burma:-
GDP (combined) : $2,518,435 million (2014 US Dollars)
Population (combined): 1,638.2 million
GDP per-Capita [10]: $1537.32 (2014 US Dollars)
%of World GDP: 3.4%
{*}
End of statistics...
In short, the Indian sub-continent, as a whole, got marginally poorer between the laying of the foundation of Taj and the establishment of the British Raj, but the economy recovered and actually improved measurably by the time Kaiser Willy threw his tantrum!
The decline in prosperity between 1600s and 1870s is attributed by my sub-continental compatriots to the
Goras robbing us poor, help-less sub-continentals blind. But if the economic parameters are anything to go by most of the GDP growth of the UK between 1600 and 1870 is quite moderate by standards of pre-industrial Western European economies [11] and past 1830s the rapid growth of the UK economy can be more like due to the availability of cheap raw materials from the colonies than to direct taxation and down-right economic banditry of the British in the sub-continent. The decline could very well be attributed to the change in administrative and economic structure of the sub-continent with the arrival of the British, their industrial goods and their new rules. But does it necessarily have their greasy finger prints of thieves all over them? That is a question to ponder.
While world GDP did increase significantly between 1870 and 1913, i.e., during the 'moderate' phase of the INC, the share of India in world GDP actually grew! I cannot believe that is possible without improvement in the living standards of some of the people in the sub-continent. It is possible that economic inequality might have flourished in that phase of growth but there would still be some tangible benefits to the poor in the sub-continent. I mean just look at modern economic data. Today we form a much smaller share at 3.4%, for the subcontinent as a whole, of world GDP and our GDP per capita is only about $90 better than it was back in 1913 and only about $180 better than what it was in the 1600s. For India proper the stats are marginally better with an actual GDP per-capita being only a $100 more than what it was in 1913. Wow! A hundred years to raise the GDP per-capita by a 100 dollars!
So wait India hasn't really gotten richer but rather has gotten kinda poorer since independence?[12]
[8] source: Angus Maddison. World Economics. Vol 9. No.4. October-December, 2008. -Thank you, Google!
[9] I know its bit of a guesstimate on my side but bear with me please. source:
INDIA: population growth of the whole country - Thank you again, Google!
[10] Whole sub-continent combined: India+Pakistan+Burma+Sri Lanka+Maldives.
[11] UK GDP growth in
1600s is
0.76%.p.a.,
1700s is
0.58%.p.a.,
1820s is
1.021%,
1870s is
2.055%. Values for Western Europe for
1600s, 1700s, 1820s,1870s, are,
+0.396%.p.a, +0.214 %.p.a., +0.566 %.p.a., +1.679.p.a., +2.111.p.a, respectively. For Western offshoots (USA, Canada, Australia, etc.,) for
1820s and
1870s are
+2.348 %.p.a.,
+4.313 %.p.a., respectively.
[12] Is it just me or isn't decolonization supposed to have benefited the former colony more?