I noticed while creating a new post that if I provide a Thumbnail URL with an external (not hexbear.net) URL, then the original image gets used when viewing the post. I first noticed it on this post where I added a youtube thumbnail icon (check it in the browser console).
I tried researching if there's a bug for this in upstream Lemmy:
This one seems like it would have fixed it: LemmyNet/lemmy-ui: Update post listing to prefer local image when available
And see also this: LemmyNet/lemmy: Add initial skeleton of image proxy improvements for feedback
Unfortunately I'm not that familiar with the Lemmy codebase, but how possible would it be to proxy+cache thumbnails? I don't quite mean store it permanently in the database, just proxy the source image and cache it for performance reasons. Otherwise, maybe we should disable the thumbnail URL for now until this support is added upstream?
IMO the cost is immaterial. It is a design decision. Less caching makes the start-up cost of instances lower. More caching makes the instances safer to use, but increases the requirements of running one. Either choice could be correct depending on the context.
'Balloon' in this case is still manageable. In my case, this storage still costs less than $20 a month, (the minimum is about $8/mo for 1TB or less). If you're paying Amazon prices you're fucked though.