Good thinking there pal, but you are missing the point. If I wanted to store 25GB i could probably find a spot that'd do it for free (Mega, Google Drive etc)
If you need free space to store something this is so not for you buddy... Just go register 10 google accounts to spread your stuff around and you never need to pay for storage;) some people like to be a customer not a product
Yes, it's seriously overpriced and storage costs hasn't decreased on Tarsnap for ages. That is the reason I am currently migrating off Tarsnap after using it for over ten years.
No. I already said it's probably not for you. If you need lowest price per GB and don't care about anything else including your time or customer support then nothing beats a bunch of google accs with free drive tier. No price is better than zero
But for me tarsnap is convenient as extra offsite backup for valuable stuff. Price per GB is not the only factor and different people have different priorities.