Dear Ranjan,
Go through the following -
There is no specific provision to find out the cost of such converted non-agricultural land. Hence, the cost of agricultural land would be the cost of non-agricultural land.
A conjoint reading of sections 2(42A), 47(iii), 49(1) and 55(2) of the Act suggests that, in law, no "transfer" of a "capital asset" is considered to take place on inheritance and succession. The liability for capital gain arises only when the capital asset is actually transferred by the successor. It is only when the ultimate successor transfers the capital asset for a consideration, the capital gains are assessed to tax.
As you get the land as gift ( not paying any consideration ) the question of capital gain shall not arise.