I agree with the rest that there is usually a very minimal difference between free SSL and premium SSL in terms of providing data transfer encryption is concerned. However, the difference comes when it comes to the expiry period. For let's encrypt SSLs, one has to reissue them after 90 days while a premium certificate takes a whole year before one reissues.
Different SSL certificates are designed to cover different needs of the clients, for example, there are those that cover the main domain name and addon domain names, or one main domain name and unlimited subdomain names. In any case, it is better to check the features of a particular SSL certificate before making the purchase decision.