I have do some searching and browsing dedicated server offers from Rackspace but their pricing is so expensive if compared with other providers as OVH, Scaleway or online.net.
Is their pricing included support and managed services?
Well, you aren't paying Rackspace just for the server. Their managed hosting is far different than what major providers offer.
Usually, most of the cheaper managed hosting providers will just provide a 24/7 support with proactive ( not reactive) security and off-site backups and all this is provided only if you are using cPanel.
Rackspace, on the other hand, doesn't just deploy a standard setup for your servers. They make sure to understand you service requirement and deploy the required architecture ( software and hardware based ) along with their 24/7 Reactive support ( They will actively search for threats and make sure breaches don't occur rather than just initially securing and patching the system weekly)
Secondly, these guys monitor your services to the CORE and lastly, most of the managed hosting providers don't consider looking into a client's PCI compliance where else RackSpace does.