I'm going through the exact same transition right now. $20/month is more than the cert itself.
IIRC it's because they can't just attach an extra IP to their load balancers for each SSL site, since Amazon doesn't allow you to attach multiple elastic ips to one instance. So, they need to use an elastic load balancer for each site, which costs $18 per month + bandwidth. So they are probably actually just breaking even or losing money on that $20/month fee.