The cost of the online form is Rs 200. You need to fill out the submission form online and make an online payment. After the payment you will get a 'transaction ID' and you will be then directed back to the Tata Nano website where a Unique Identification Number (UIN) will be generated.
After booking is complete, you have to take a print out of the receipt after which an email is sent to you with the receipt. Please note: you cannot use Credit cards as they will not be accepted for payment of booking amount. So you need to have internet banking facility for booking the Nano.