Its not so much the cost of the server hardware as it is the cost of ongoing maintenance, support, hosting (data center), ect. But even at that, still very poor reason to not offer dedicated servers for certain parts of the game (like crucible). The cost is not prohibitive like it would have been in 1995, which is where their technology mindset seems to be (right there with "UDP is faster over the Internet"). They don't even need dedicated hardware, just spool up more VMs from Azure or Amazon on demand.