However, if you choose to secure a subdomain only:
For non-Comodo certificates, this feature works only when the certificate is activated for domain name -- not a subdomain.
PositiveSSL will secure both www.sub.domain.com and sub.domain.com
Not 100% true.
a positive SSL will work on the main domain and anything you use as www.domain.com/sub or domain.com/sub
if you try www.sub.domain.com all you get is 'Your connection is not private -NET::ERR_CERT_COMMON_NAME_INVALID'
if you want to use www.sub.domain.com under SSL then you need to use www.sub.domain.com as the common name
i know as a reseller Positive SSL is far cheaper than a Rapid SSL, so can easily be bundled with a hosting package without extra costs
But that isn't really the point of this thread . The point is to discuss why the prices are different, what is different between them that makes 1 more expensive than the other.