The cost to users (and especially developers) most assuredly is not simply 2x that of a single host. Planning for full replication and hot-failover from day one adds complexity, and can actually do more harm than good if your team and tools aren't fully up-to-speed on the sometimes subtle challenges surrounding replication lag, eventually-consistent stores, and STONITH.

My thinking was, if they already own a datacenter in another location, why not add another floor? Obviously it's not cheap but is it really 2x? If you're big enough, you don't have to hire double the staff or buy double the resources since you already have them.

The expensive part about data centers isn't the square footage, it's the HVAC/power/bandwidth/servers. So you have the space, that means you still need to reserve twice the bandwidth, power, cooling and hardware. x2 cost. Actually somewhat more because you need to account for all the bandwidth and time to keep everything synced up.

at what rackspace can charge, new hardware is nothing. the problem is that if I rent you a server, it's going to be pretty difficult for me to replicate that server to another location without a lot of cooperation from you. the more I screw with your server, well, the more likely I am to break it, too.

It's more than that as you two datacenters, plus the cost of synchronizing it.