MS offer it for free though I am sure.
Publishers/developers still have to pay for them but they are MASSIVELY cheaper than getting them from other companies. The main advantage being that Azure is all tied in to Xbox Live so, I'm assuming, it's a lot easier setting them up.
This is a quote from Respawn about using Azure for TitanFall
Most importantly to us, Microsoft priced it so that it's far more affordable than other hosting options