Parking sensors are good enough. The sensors are drilled into the rear / front bumpers, you can get a display or no display sensor kit along with it.
The sensor detects the distance of objects in the rear / front (depending on whether you have front sensors) of the vehicle, and throws an audible warning sound (beep beep). The beep gets stronger as you get closer to the object in the rear / front. Hence its immensely usefull in tight parking spots and the present busy work schedules.
Installing the parking sensors will be definitely cheaper than a camera system, by a few 1000 rupees.
----------------------------
DX
-----------------------
Parking Sensor-Parking Sensor