You need to separately assess your needs in a) serving content over SSL, and b) having a 'vanity URL' or custom domain as you write (i.e.
There are plenty of cheaper CDN providers who will do SSL on their own domain name, i.e. something like
A CDN typically has quite many web nodes (webservers), perhaps in separate clusters. When serving HTTPS, each of the webnodes that can be hit with a request for your content must have the certificate installed. That's an essential part of the authentication that SSL provides. Thus the handling all these customer certificates leads to a fair amount of work for the CDN provider, and that tends to be reflected in the price...
Bottom line, I doubt you will find SSL with vanity domains much cheaper than what MaxCDN is quoting you. You could try looking through all the dedicated re-sellers of established CDNs like Akamai, EdgeCast, CDNetworks et cetera. It might yield something, but it will likely take some time.