What will be the cost of repair ............. any rough idea ????

If it is under warranty and it is decided it is a defective part they will repair it for free. If they think it was user damage such as you dropping the computer they will not repair it for free. In a few cases where a common defect was found in many computers at a much later date then there may be a special notice about free repairs. Otherwise the cost is borne by you.

I took my first generation Macbook Air in to the Apple Store today, as the hinge just cracked two days ago. They quoted $280 total to repair the hinge - $100 for labor, $180 in parts. Although this was a known issue from Apple, and they offered to refund others who did not have AppleCare (I had the 3 year AppleCare), they refused to pay for the repairs, as it was past the 3 year mark.

Repair what, exactly?