how much is the custom duty of importing a product

The short answer is 42.5% of CIF (cost, insurance and freight) as all personal imports come under HS98049000, and are charged at flat 28% GST (achhe din aa gaye na!).

Sometimes you might get lucky if assessing officer charges GST at a lower rate, or does not include freight(shipping) in CIF. Both have happened with me. Be warned that Fedex/UPS/DHL/Aramex levies brokerage charge and flat convenience charge above this.

However, if your seller uses DHL Saver (used by bhphotovideo, sometimes also by parcel forwarders like myus), which goes via Frankfurt airport and then gets handed over to IndiaPost, then you might avoid duty as USD50 falls below Rs. 5000 and postal dept usually does not charge duty on such low value items.