Bonus shares are issued at zero price. The average price of your shares reduces after receiving a bonus because the number of shares you own increases without any additional cost.
For example, if you had 10 shares of a stock priced at ₹100 each (total ₹1,000), and the company issued 1 bonus share for every 1 share you own, you would now have 20 shares. However, the price per share would adjust to ₹50, keeping the total value of your investment at ₹1,000.
Note: On the ex-date, the stock price drops because more shares are available due to the bonus issue. However, your total investment value stays the same because you receive more shares. The stock price and 52-week high/low are also adjusted to reflect this bonus.