Well I donno how much the exact cost is but I can find out. But the best thing for you to do is to go to the Pre-Paid Taxi counter in the Delhi Airport operated by the Government and hire a taxi to take you to Agra. There will be other private operators as well but the government counter will be the cheapest. I assure you they are safe. When are you planning to travel?

I dream of a better tomorrow where Chickens can cross the Road without having their motives questioned - Unknown