Insecure Connection from mixed content with SSL, Domain Mapping, and Multisite

I just divided a large site into many smaller, more efficient sites via multisite (domains). This worked out great, until I wanted to have an SSL installed to boost page rankings (as a plus, secure accounts). I don't do transactions on-site, only through PayPal, so it wasn't a needed item. But since my site is growing so quickly, and Google announced the SSL-rule for rankings, I wanted to ensure a great experience for everyone.

I have two mapped domains currently, with more to come, and I feel I am destroying user trust due to the "hacker" alert that comes from the SSL being signed by a different domain name (my main site on WPMU).

Is there any way to attach a separate SSL to my mapped domains?

I have very little knowledge of SSL certs, however with a little research, I found that a Wildcard may solve this issue? They're so expensive though, and my site does not quite generate enough income to afford that. :slight_frown:

Is there another way to get around this?
Maybe have the SSL only show on the main site and not mapped/subdomains?

Any suggestions are very welcome, as I can no longer think further than my current knowledge and capacity. Seeking to learn more about this issue!