I don't know if this works for your clients, but, if you can get a yearly fee out of them, it might work well , and in your favor.
Consider:
"For up to 100GB of digital storage, we charge a yearly backup fee of $X ; you will be billed yearly for this." (for more, you either give them an expanded price list or a custom quote)
So consider that online services such as Microsoft OneDrive or Google charge about $10/month for 1TB or 1000GB of disk space, it works out to $120/year for 1000GB.
Thus, for 100GB of space, your cost is $12 per year.
I would guess you should easily be able to charge between $50 and $100 per year for this service - pointing out that (MS or Google) have super large networks and redundancy of all hardware. MS OneDrive even claims to be HIPPAA (healthcare privacy level of security) compliant, I think.
So now every 12 months, your billing software will kick out a bill for them. Over time, your storage costs may drop; but you can charge the same (everything else is going up but you are not charging them more).
Hypothesis:
50 clients @ $50 each (after 1 year): $2500/year revenue. Your cost to provide (which is integrated into your backup/workflow automatically) is $600. Over time this could grow, as you add more clients, into a nice little dividend each month (since customers will pay you in advance for a year, based on the month they started with you).