All the subs can actually carry 12 Brahmos. Even the 950T version of Amur has been advertised with 8 Brahmos VLS.
That isn't the cost of the sub alone, it includes the cost of R&D, production facilities, training, and also maintenance for decades. The actual sub itself is likely less than half.
It's doable with local steel, AIP, weapons, CMS etc. I won't be surprised if IN asks for all or most of the electronics to be Indian anyway.
What is stopping us from designing indigenous submarine ssk i never understood.We build aircraft carriers and ssbns,but no diesel subs wtf.