RapidSSL Wildcard Certificates allows you to secure unlimited number of sub domains on the same domain name with a single certificate. RapidSSL Wildcard is low cost and easy to manage.
Secure an unlimited number of sub domains on the same domain name. With a Wildcard SSL certificate issued to *.yourdomain.com you could protect
www.yourdomain.com secure.yourdomain.com etc.yourdomain.com