The only problem is if those subdomains have a password input field "somewhere" on their sites, AFAICT. Or are you referring to google's "later" plans to add a red exclamation point "insecure" flag to the url bar for all HTTP pages?

I'll admit when I first heard of them flagging HTTP password fields, my instinct was to write a little javascript to "mimic" password input field behavior (and store the real password away somewhere else, then at submit time, it sends in the correct data). But if it's just a tiny warning on the url bar, meh, not sure if I care...

Also note that https://letsencrypt.org appears to offer free CA certs.