There are no official reasons for the Nanos catching fire but according to some reports it its the engine and the wireing that was malfunctioning and sparking the fire. Tata has taken all steps in rectifieing the issue.
New Tata Nano with EasyShift is available at Rs. 2 lac (Ex-showroom, Delhi), and the final cost depends on the valuation of your car. So, first get the valuation of your car from authorized Tata dealer, so he can give you a final quote for the new car.