you can run a resequencing relay good enough to feed an atproto appview pretty cheaply - it just needs to subscribe to a few thousand websocket endpoints to get a live tail of the whole network (incoming traffic is well under 50Mbps at peak in my experience)

running an archival mirroring relay is storage-intensive (on the order of tens of TB iirc?) but only serves as an optimization (you can backfill full atproto repositories straight from the relay instead of needing to reach out to the relevant data server)