Free certificates tend to result in ugly warning messages in browsers …
Cheap certificates are available, however, they are still not for free. And hosting more than one domain with SSL is a problem too with most hosting providers if you do not want to book additional hostings.
> Free certificates tend to result in ugly warning messages in browsers
StartSSL is free, and as long as you correctly bundle the intermediate cert (something you have to do with many, many other CA's anyway) your SSL will look no different than a $100+/year one from an A-list provider.