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.