Since it is an ancestral property and was transfered to your name via inheritance, the cost of acquisition shall be the cost to the previous owner. If the property was purchased prior to 1981, then market value will be for the year 1981. The indexation shall be on 1981 if it was purchased before 1981 or else the year in which it was actually purchased. 

The sale proceeds will be deposted in captal gain account, and later you can invest in infra bonds or purchase another property. You will get exemption u/s 54. 

Strange, I went to Ballabgarh, the municipal and other people who knows says there is no land prices available before 1981,  and i could not find a valuer, what price should I take? Can I just file my returns right now as date is nearing and later see if I get any data and then revise if required or I can just decide my market rate and relax.

UTTAM sir, if u r not able to get value at the time of purchase,u can date value back at 1981 by reverse indexin..as in u shall take the market price of the house as on the date of sale..divide it by the cost inflation index and multiply by 100...u shall get the fair idea of the value as at 31-3-1981..i hope this will help u!!