Hi, you could have blog.yourdomain.tld and yourdomain.tld in the same certificate for free.
edit: to make it simple you have the right of one sub domain and the root domain without subdomain. As soon as you need more than one, you will need to go for a wildcard (which is not free).
edit: to make it simple you have the right of one sub domain and the root domain without subdomain. As soon as you need more than one, you will need to go for a wildcard (which is not free).