Tata Motors not disclosed any information regarding top speed of Tata Nano. You have to wait till its launch. Tata Nano has a 623cc gasoline engine that produces 33PS power so it is expected that it might reach the top speed of 110kmph.
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.