Development agreement from developer

You can consider circle rate from sub-registrar office as of 2006. 

For flats received through a development agreement on the client's land, the cost of acquisition can be considered as the value of the land proportionate to the flat received, as per tax rules.

To compute capital gains on sale, the cost of acquisition of the flat can be determined based on the proprotionate value of the land attributable to that flat at the time of the agreement in 2006.