SSL certificates that most web browsers can accept without grief are sold by a relatively small number of companies. That’s because the major web browsers are shipped with a certain set of “root certificate authorities” that they trust… and if your certificate isn’t signed by one of those authorities, or by a certificate “chained” from one of them, then you’re out of luck— the web browser will display a scary warning to the user or, in some cases, refuse to work with your site at all.

The cost of SSL certificates varies quite a bit, from as little as $20 to as much as $1,000 or more. Why such a big difference? There are three main reasons:

via WWW FAQs: Which SSL certificate should I buy?.

2. Some certificates are directly signed by a trusted root certificate, while others are “chained” from another “intermediate” certificate. This isn’t really a problem, as long as the company selling you the chained certificate really does own the root certificate. But some webmasters get confused by intermediate certificates, fail to install them correctly, and mistakenly think they have purchased a bad certificate. So chained certificates are usually less expensive to allow for this inconvenience, even though there is no real technical disadvantage.