latest pricing at DEL airport:
1000 rupees for airtel sim for the month
700 rupees for Vodafone for the month

registraration quick at airtel kiosk - people seemed friendlier at Vodafone - airtel activation happens within 4 hours or so after buying the SIM card - no passport photos or passport copies of own required - they take your photo with their camera and copy your passport - no hassle at all

SIM cards like above are available for just a couple/few hundred rupees at shops outside the airport but it is likely worth the increased cost at the airport for the seamless purchase and activation process for foreigners