"Indigenous" S-400 will be under what is known as "Project Kusha".
From what I heard, a major advantage will be the standardisation of the kill vehicle components (seeker, warhead, fuses, other components) i.e differences will be only propulsive scaling for IAD tiers....for say 3 missiles similar to family format like ASTER etc.
This will be major advantage (production wise anyway)... to current S-400 missiles in Indian IAD being (underlying) different to each other: 9M96, 48N6 and 40N6
^^ Is this what you have come across @Gessler ? et al.
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40,000 crore INR ~ 4.7 billion USD
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Bharat Electronics Eyes Rs 40,000 Crore Order From Indigenous S-400 Project
Bharat Electronics and DRDO are jointly developing key radar and control subsystems for Project Kusha, with order potential hinging on system readiness and user trials.www.ndtvprofit.com
There is concurrent development to essentially bring about an "S-500 equivalent" in project kusha

DRDO Chief Confirms Project Kusha Equivalent to Russian S-500 System, Surpasses S-400 Capabilities - Indian Defence Research Wing
SOURCE: RAUNAK KUNDE / NEWS BEAT / IDRW.ORG In a significant revelation, Dr. Samir V. Kamat, Chairman of the Defence Research and Development Organisation (DRDO), has stated that India’s indigenous Project Kusha air defence initiative is not merely comparable to the Russian S-400 Triumf system...

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Original (Hindi only) source interview with. DRDO chairman Dr. Kamat, proj. kusha mentioned around 7.30 mark: