RapidSSL will provide encryption as will any SSL cert (it's possible that a server-client connection could end up using null ciphers under some circumstances, but that's not down to the certificate)
Additionally in some environments, self-signed SSL certificates which are free are a perfectly reasonable option.
The difference you're seeing in cost of certificates usually comes down to level of checking that's done by the certificate authority and other "features" provided by the certificate, like what it can be used for and whether it's a wildcard certificate.
Cheaper certificates tend to not involve a lot of checking by the CA that you're actually entitled to a cert for a given domain, with some it can be as little as being able to receive an e-mail to certain pre-specified email addresses at a given domain (eg, ssladmin@domain).
The theory is that the more expensive certificate with more checking is better because users can have more trust that it's legitimate. However hardly any users ever check the certificate issuer, so I'm not sure that's a valid reason.
The primary thing to watch on certificates for public services is that the root certificate used to sign yours is embedded in all the major browsers. If that isn't the case your users will get certificate warnings when connecting to your site which may deter them from using it.
There is one exception to that which is the use of "EV-SSL", which is quite popular with banks and some larger e-commerce sites. an EV cert will provide some visual notification in the browser that it's in use which is designed to make the user feel that there's more trust in that connection.
To get an EV certificate the issuing CA should be doing quite a few checks to ensure that you are the appropriate owner of the certificate, which is one element of why they have a higher cost.