I was comparing the costs to building/operating on a single server to the real Twitter. Twitter is a massive, distributed, hugely complex system, which requires

I was comparing the costs to building/operating on a single server to the real Twitter. Twitter is a massive, distributed, hugely complex system, which requires a large team to build, maintain, and operate. That costs $$$. Both for people and servers. (You are right, servers are cheap compared to employee costs though.)