How much should I pay when the EC2 instance is turned off? I don't need the instance running 24x7.

You pay only for the hours that the machine is up and running. If you plan to reboot the machine frequently, we recommend you to use an elastic IP so your IP does not change all of the time. This tends to confuse users and to create edge cases in the instance configuration that night eventually break it. Using an elastic IP is also a good first step towards using an organized domain to host the instance (instead of an Hossted domain).