Here's a good excuse for not using https for everything: it breaks caching of files by proxies!
Serious question, what are my options?
* Make use of edge CDNs with https termination
If the former, you can stick those on HTTPS too just fine. CloudFlare will be an entire SSL-enabled CDN for you for free. Amazon Cloudfront will serve SSL for you for free (though you still have to pay for Cloudfront itself, and get a cert on your own, though you can do that for free).