If you are looking for 3G enabled dual camera mobile for video calling, than the cheapest mobile with this facility is SPICE G6500 . Its price is Rs 5500 in India.
SPICE G6500 mobile is having 1.3M & 0.3MP Dual camera. So you must not expect very good resolution with it. But you can not expect much with this cheap price.
Video calling charge is much more in India if you use 3G service.
Nokia Dual Camara Mobile Phone ( Nokia C5-00 3G mobile )
Nokia Dual camera for video calling is having good resolution camera of 3.2 MP and VGA. Its price is slight more, its cost in India is near Rs 7300.
LG OPTIMUS 3D
If you go for more higher price, LG Optimus 3G mobile is best option. It is android 2.2 based dual camera mobile. This mobile is just about to come in India with detailed Price, but expected price of LG Optimus 3D mobile in India is Rs 30,000.