50GB is wild. I don’t think images that size even deploy on Fly?
As far as I know, image store is free, and Fly only retains a handful (probably just one) image for v1 Apps, and deletes the rest.
Things might change once Machines (Fly Apps v2) are generally available (since v2 Apps can be deploying any number of variety of images): We're launching Fly machines today - #8 by starptech
Proposed pricing isn’t anything too drastic (even if a tad expensive), at $0.15/GB per VM per month.