[Firebase founder] I would caution against comparing our pricing to S3 or any other non-realtime storage service. Firebase's use case is primarily for serving structured data (the type of data that lives in a database). For most apps, this is a small piece of overall data usage. Your site probably spends most of its bandwidth usage on downloading static content -- HTML, CSS, images, videos, etc -- which you don't need to store in Firebase (you can, but if you're looking for a cheap way to store big video Files, Firebase is not the answer).
Also keep in mind that for most apps, 90%+ of infrastructure costs come from running servers, not bandwidth. So while we may seem more expensive than other services, we eliminate your costs for servers and backups. The only thing you pay is the per-bandwidth/storage price. Also keep in mind that Firebase apps can often use less data per-user than traditional apps, since they push data only when it changes, rather than polling.
The vast majority of the apps on Firebase today will fit nicely in our free plan.