You seem to be unfamiliar with the genesis and the history of the Indo-Russian relationship. Since I have been around on this planet to see some of it, let me try to explain in a very short way.
It started way back, and simply because India did not choose to be part of any camp, especially part of any anti-Russian camp. While what everybody here (including you) seems to be more taken up with the Military aspect of the relationship, actually it went far beyond that and had a bigger economic dimension. In those days India was short of hard cash (esp Dollars) and the West was not particularly enthusiastic to pass on technology to India. That is when the Russians stepped in. They helped to grow India's steel industry, helped with Indian Oil exploration and Refineries. So much so that when India nationalised the American Oil companies (which were bleeding the Indian economy, as well as most of the world, btw) the American oil majors promptly shut off crude oil sales to India. That is when the Russians stepped in and supplied crude to India. But except one refinery, the other Indian one were designed to handle American supplied crude, so the Russians helped to modify them to handle other crudes, while this was being done; they supplied petroleum products so that Indian industry would keep running. And so on.
Now the question was payment for all this. Deferred payment being part of the deals helped but was not a complete solution. So the Russians accepted Rupee payments partly (though the Rupee was not worth much then), but most innovatively they accepted payment in commodities. And of course there were grants. It was a big combination.
Around that time the transfer of arms hardware and technology started too. And the same innovations were used. The first Indian Foxtrot class subs were paid for by Indian banana exports and T-55 tanks got paid for by leather boots for example. And the Russians helped in any (and every) way possible. When Indian Air Force needed medium transports they looked at AN-26s; but they were not suitable for the hot and dusty weather and high altitude operations required in India, so the Russians modified them for the IAF as the An-32s and India bought them at less than $1 M apiece (an unimaginable price). So India bought over a hundred of them. Now those birds are fully amortised (long ago), and continually upgraded and will see service for at least a decade more.
Times have changed now, India has much more money in her pockets so now India is investing in Russia. India is a big investor (and JV partner) in the big gas project in Sakhalin as well as oil exploration with Russia in other parts of the world. India has invested in Brahmos in a big way, just as Indian cash has helped to revive parts of the Russian ship-building industry. Not least to mention the Russian nuclear sub construction (the Akula class). The clock has turned somewhat and India does not need to go hunting for "bargain prices" not least of all from the Russians. Now everything is negotiated with the Russians as with anybody else. And BTW, Mr. Lakshmi Mittal (who now happens to be the biggest steel company owner in the world) and his family cut their teeth around the Russian built steel plant in India. Now he has helped to prevent (among others) Russian steel plants from closing shop and thus preserved Russian jobs. So good begets good.
But the history endures. Just as the Indo-Russian relationship does, and in a much more evolved form.