SSL means Secure Socket Layer widely used in World Wide Web. In order to support SSL on Web Server, you should purchase certificate that compatible with known commercial browsers such as internet explorer, chrome, opera and so on.
The below website sells SSL certificate in cheaper price
In my case, I prefer to use that certificate on multiple domain, and they offer cheapest one at following URL
As of Dec 12, 2018, the cheapest SSL for multi domain is USD 15. Isn't it cheap?