I assume "couple of" was figurative, to indicate the cost is substantially less than managing your own bank of SSDs and ensuring it is periodically powered etc.
[Edit: LOL, I see someone else posted literally the same example within the same minute. Funny coincidences.]
That said, they could also be storing relatively small amounts. For example, I back up to Backblaze B2, advertised at $6/TB/month, so ~300 MB at rest will be a "couple" bucks.
> managing your own bank of SSDs
If I have enough data to need multiple SSDs (more than 8TB) then the cloud cost is not going to be substantially less. B2 is going to be above $500 a year.
I can manage to plug a backup SSD into a phone charger a couple times a year, or leave it plugged into one when it's not in my computer being updated. Even if I rate that handful of minutes of labor per year at a gratuitous $100, I'm still saving money well before the 18 month mark.