The Rackspace CDN isn’t too complicated to setup. If you wish to configure the CDN with an origin dedicated server, there is really only one main requirement;
For example, if the CDN domain rackspace gives you for use with the CDN product is cdn.customer.com.cdn306.raxcdn.com, you will be required to update the website configuration (origin) for Apache on this server.
i.e. you need to include the CDN domain cdn.customer.com.cdn306.raxcdn.com as ServerAlias for Apache or as Server_Name for Nginx. Then that server alias/virtualhost receives and handles the request for the CDN.
Basically for CDN request comes in to rackspace raxcdn.com domain, and if there is no cached version on a local edgenode, the CDN makes a call to your origin server IP configured, sending a HTTP header hostname. This is indeed why you need to setup a serveralias for the rackspace.raxcdn.com hostname, once it is created.
Provided that files can be accessed on the IP address of your origin dedicated server. i.e.
When request comes into http://cdn.customer.com.cdn306.raxcdn.com it will if not cached try the origin server.
If you wish, you can add an additional CNAME of your own for your own domain.
i.e. cdn.mydomain.com CNAME cdn.customer.com.cdn306.raxcdn.com
This allows customer to make request to cdn.mydomain.com/mycdn/images/profile.jpg which forwards to Rackspace CDN, and if it has no cached file, it will forward to origin server the request, and cache the file around the edge nodes. The edge nodes are transparent, and the only thing you really need to worry about is configuring the virtualhost correctly.
The only reason you require the virtualhost, is because the CDN uses hostheader to allow your server to identify the virtualhost serving CDN. This naturally allows you to serve CDN on the same IP address. Naturally, if you intend to use SSL as well you may wish to consider a dedicated IP and find this community article of use.
Rackspace can help you configure the virtualhost and potentially may even be able to help you configure CDN product with your dedicated server origin as well.
For pricing, please see the CDN calculator at;