HTTPS as a ranking signal – google!

Now that Google has announced that *HTTPS will be a ranking signal, what would be the best way to implement SSL on subdomain structured Multisite install? In my case none of the mapped domains have shopping carts (at this moment) so would a shared self signed cert be sufficient using Apache’s SNI or does each need their own store bought SSL or wildcard? Thanks.

* http://googlewebmastercentral.blogspot.com/2014/08/https-as-ranking-signal.html