Tata Nano is 21% more spacious & Giving about 20% more milage then Maruti 800. And yes the Cost Factor is there. But You can get the Maruti on the same day when you decide, But for Tata Nano, You have to Book it in Rs. 70,000 and wait till your turn.
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.