Now airtel is offering generous fair usage policies with a limit of upto 150 GB in its 4mbps plan costing only Rs. 2099. After your download limit of 150GB your speed will be reduced to a mere 256 kbps but that is now quite reasonable with a 150 GB limit which was earlier 50 GB. For more this discuss here at IBF.
Here is a summary of the new plans compiled by teleguru.in :