First a clarification: there are no subdomains in the way you are using - only domains. Or better you can say that any domain that you will own is a subdomain. The root domain is ".". The TLD "com." is a "subdomain" of ".". "example.com." is a subdomain of "com."... A subdomain is a domain defined inside another domain. but this is relative, not an absolute attribute.
The wild card certificates are more expansive not because they are different from a domain one, but because of their exposure and chances to get compromised. You are not paying the SSL CA the "price" for certificate, but an limited insurance. This insurance covers only if the breach is caused by a mishandling of your certificate and it's chain by the CA.
If you have only a few subdomains it is cheaper to buy a certificate for multiple domains (certificates that use Subject Alternate Name). If you have many subdomains of a domain or you expect to add an unknown number of subdomains you should better buy a wildcard certificate. If you have different domains (example1.com, example2.com, example1.us) you can use only SAN certificates or you buy a wildcard certificate for each domain. (E.g. you can not buy a wildcard certificate for *.com).
Using SAN certificate or wildcard certificate could lower the security of your configuration, because will force you to use the same listener and most probably use the same user (you can run with different users with something like mod_suexec for apache). So if one site gets compromised, could lead for the other sites to get compromised. If you have different certificates you can run those applications as different users and have better security.