Yeah that was my sort of chain of thought.
On the assumption I’m the first person to have considered doing something like this (on this forum anyway), late last night I signed-up to Dropbox’s £7.99pm plan for 1TB online storage with a view to being the guinea pig. There was an option to pay £79.99 for a full year but I want to be able to opt-out as this is initially a feasibility study, so I chose monthly.
Starting just before 1 o'clock this morning I deleted all the contents from my existing Dropbox folder on my Mac and allowed it to synchronise. Next I opened Dropbox preferences and redirected its home directory to my NAS, where it instantly created a reassuringly-empty Dropbox folder. Then I dragged the entire iTunes folder into the Dropbox folder. The transfer was of course immediate because the iTunes folder was already on the NAS, and so from a file-handling perspective nothing physically moved on the drive, just a quick update to the TOC.
And so began the automated Dropbox syncing process, shortly before 01:15. I think it reported a total just shy of 15,000 items, which will be about right. My CD library runs to approx 11,000 songs all lossless, plus all the accompanying subfolders by artist, the actual library itself and the artwork. I won’t be at all surprised if this takes several days. Once everything has copied across, I’ll let you know how long it took. Then I’ll simulate a disaster-recovery scenario by disconnecting the NAS and instructing Dropbox to sync to a new location, and I’ll give you some feedback on that.
If everything goes to plan, I'll then move my ripped films into the Dropbox folder, which probably will take just as long; only 100-150 files including folders but the films are of course an order of magnitude larger than my lossless audio files and collectively are of a similar size.
Interesting experiment, if you are so inclined, of nil importance if this kind of stuff does not interest you, but all I can say is never underestimate the assurance of keeping clones of your eggs in many baskets. Dropbox isn’t really meant for backups, but at 22-26p per day depending on if you pay monthly or yearly it's an affordable Plan C if this experiment works.