if you generate your SSL Certificate by yourself, HTTPS/SSL will work, but a browser will issue a warning encouraging the user to not trust the site, as there is no way to tell if the site you are accessing is really who they say they are. So you need an authentication from a Root CA to avoid this problem. To get this authentication you need to pay.
But now the things are changing. You could read about the Let's Encrypt initiative; its aim is to provide a free CA. It will generate legitimate certs that are trusted by a significant percentage of browsers.
