Bitcoin price is dependant on multiple factors that govern the demand to a limited supply commodity, bitcoin.
Here’s an article that does explain in great detail what and how bitcoin prices are governed:
Bitcoin price varies from exchange to exchange and wanting to buy bitcoin for a cheap price means that you’d have to track every single exchange for the cheapest price of Bitcoin.
Unocoin offers ₹200 for free when you sign up as an incentive for acquiring customer to their clientbase.
Notice how Unocoin has different buying and selling price in India. They buy at a lower price and sell at a higher price.
In the article, notice how they talk about different pricing due to induced geographic demand. Buying Bitcoin from a country that isn’t inclined to buying Bitcoin as much as selling would be a worthwhile trading option.
Coinsecure offers around 300 Rupees per customer acquisition, but I personally haven’t tried it out and wouldn’t be able to vouch for it.
Coinsecure does have lower margins when it comes to the difference between buying and selling price compared to unocoin.
Local Bitcoins in India is another alternative to buy Bitcoin at a cheaper price. Local bitcoins involve individual people bidding and selling Bitcoin interpersonally for cash, commodities or anything else.
I suggest you wait for at least 15 confirmations prior to trading just to make sure that the transaction goes through and you don’t get scammed.
These are three main and legal ways through which Bitcoin can be bought for cheap in India.