Information is mainly drawn from http://www.ndtv.com/article/india/ndtv-exclusive-this-is-ins-arihant-first-made-in-india-nuclear-submarine-578949?curl=1408597764 and my knowledge of Indian subs since 2009 with many previous articles on this blog concerning Arihant..
The nuclear sub design evolutionary process was particularly difficult for the US, Russia and China involving many test nuclear subs. As the UK and France received direct or indirect assistance from the US their evolutionary stages were far briefer involving fewer test subs. Much of the money India paid to Russia to modernise India's other nuclear submarine INS Chakra (the ex Russian Nerpa SSN) was for Chakra. But much of that money has also cross-subsidised Russian assistance to accelerate Arihant's development. A good strategy.