Value of right may be calculated as :-
(Proportion of Right Shares / Total Proportion of Shares) * (Right market price of each share - Right issue price per share)
Example: A company offered to its shareholders the right to buy one share of Rs. 100 at a price of Rs. 150 each for every three share of Rs. 100 each held. The market value of each share was Rs. 200.
Proportion of Right Share is 1 against every 3 shares held OR 1 Right Share out of 4 (total) shares
Therefore, Value of Right = (1/4) * (Rs. 200 - Rs. 150) = Rs. 12.50