I do not know about Chennai, but will relate my experience after arrival in DEL. The arrivals area AirTel shop (run by a franchisee no doubt) demanded Rs 660 for a 28-day connection. Luckily I smelt a rat and walked away; got a prepaid taxi at the airport to my hotel (the yellow Delhi Police prepaid taxi counter 30 feet away across the road from the exit door) was substantially cheaper than the two prepaid taxi counters) and then got a SIM from town for about Rs 280, activated in a couple of hours!