Cost of extra Baggage on flights

Just spoke to a few Airlines to find out the cost of taking extra baggage ( above permissible limit of 20 Kgs) on flights and found out that it was frightfully expensive,  It is much cheaper to Airfreight it using a reliable courier like Blue Dart. As a comparison, extra baggage on Air India costs Rs. 300 Rs. a Kg for Bangalore - Delhi vs Rs. 50/- per Kg on any reputed courier