Storage is cheap even when you need high volume. It’s literally the cheapest component in the architecture. High performance, in this context, is about replication and caching. Replication is expensive because of network utilization, caching is expensive because of the high demand. Neither are really a function of how much content is stored but about how much content is demanded. If I were to guess, likely less than 10% of all of YouTube’s content makes up 95% of its costs.