Storage and bandwidth are pretty negligible. I'm under Digital Ocean's quota for bandwidth and NewsBlur is pretty aggressive in keeping feeds up to date. I don't keep an archive of every story ever published, but most feeds don't publish all that much and I could probably invest in storing everything with a cheaper archival DB.
Although the 80/20 rule is in effect, and when I query the DB I find that 80% of feeds have only a single active subscriber and 20% of feeds have 2+ active subscribers (activity in the past 30 days). But that's still quite a bit one-to-many fetching savings. And the majority of fetches are 304s.
The newsblur dev is in this thread and he runs a good reader and could probably say what the storage and bandwidth was.