Valuation of sale of leasehold rights

but having problem with cost of acquisition as the rights have been acquired in 1972 and they were paying 15 rs per month and had only 500 deposit. 

No. Only rent was payable on leasehold rights. later on they constructed buildings on the land. Now individuals are wanting to sell the rights along with buildings to a company.

1. Leasehold right is nothing but a right over that property for an agreed period. The lessor can use this property in whichever the way he wanted as per the agreement lessor and lessee. 
2. However, this rights can be sold to another person for a consideration and such a person would then acquire the rights over that property. 
3. Sale of leasehold rights is taxable as capital gains and whatever the cost paid to it will be considered as the cost of acquisition. 
4. Now if the building is constructed on such leasehold land it is better the same is sold separately because of the reason that building is constructed by the lessee (if so) and such sale would amount to transfer of full ownership and it will not be considered as sale of rights. Even the land belongs to the lessor but building on top of it is constructed by the lessee. 
5. There is also another interpretation with respect to the above case. Though land belongs to the lessor any building constructed will also belong to the lessor after the expiry of the lease period. 
6. If we go by the above interpretation building cannot be sold in the name of the lessee as the rightful owner of the building is lessor and not the lessee. 
7. However, the sale can be restricted only to leasehold rights and it cannot be stretched to sale of the building. 
Please correct me if the above solution has an alternative view. 

Here are the facts of the case for better understanding. The individuals have already made the sale.

A company has sold leasehold rights pertaining to land to an individual in 1972. After his expiry, his family acquired the rights. They were paying small amounts of rent to the company up to a lease period of 30 years and renewed after the expiry for another 30 years. Also they have constructed a building.

Now the individuals have sold the rights and the building to another pvt ltd company with the approval of lessor company. Now how to compute capital gain on this sale, as the cost of acquisition of rights can't be determined properly.