Can this be for real?
You have already done the math: $200 would buy 4TB of storage space, and half that sum again will connect it to the Internet with an (almost) unlimited bandwidth for one month. Supposing twenty months' life expectancy for the hard disk for ease, we get around 30 USD per month per terabyte with a DIY scheme.
Since your PC is unreliable, whoever rents it for "cloud storage" would need to use a Redundant Array of Inexpensive Donors :-). Also, they'd need a complex RAID-10 architecture to cope with the fact that most people is connected with asymmetric DSLs, so they can't upload at the same speed that some "cloud user" is needing and willing to download.
All in all, for the system to even begin to be feasible you'd need to the tune of ten physical terabytes for every logical terabyte you offer, while with a "real" disk you'd be almost one on one.
And with these ten terabytes donated by the common guy, they need to compete with DIY cloud services at 30 USD per month.
Actually they're competing with even less expensive professional outfits such as Amazon and their level of economy of scale (and service level agreements. You have to undercut their prices by a hefty margin to convince someone to store their files in what I'd call Fog Storage).
So if they offer to pay more than around 3 dollars a month for a whole terabyte, they cannot be after storage space to offer to the cloud at market prices.
The syllogism (some might-be-legit options)
The above syllogism can be falsified several ways, not all of them leading to a "it's a scam" or "illegality" sentence.
if they offer to pay... : they might not offer to pay anything of real value to them (i.e., they don't have this cost), but supply some other advantage to get people to join. A fair exchange of disk space on other people's computers for example: you're their backup, they are yours. The users, not the service guys, are actually those "paying" each other. The business model then must rely on some other source of income: a participation fee, targeted ads on the file client, in-app purchases, and so on. Or they might just hope to be bought by Amazon for a jillion bucks, or become a FaceDrive.
more than: they might pay less (and use the same business model than above). Mind you, this is not money for nothing. Disk usage costs something. Extra resource consumption does not only impact "unused" disk space and "unused" bandwidth but also energy. If you earn US$ 5 per month and end up with a utility bill increase of US$ 6, their gain (they did not have to pay for electricity to keep the disks spinning and the drive heads seeking) is your loss. This is somewhat akin to cryptocoin mining: the coins you mine aren't free, they're mostly paid with increased electricity bills.
cannot be after storage space: they could be after something else. See below.
to offer to the cloud at market prices: this particular configuration of storage space might have advantages for someone, and be worth much more than standard market prices. There might also be reasons why the same configuration isn't offered by industrial services such as Amazon, so this particular option could be a seller's market.
So what might they be after (the don't-do-that option)?
Well, for the contract that turns your PC's hard drive space into a cloud storage to work, the following things should be granted and found perfectly unsuspicious:
access to your hard disk. Oh yes, only the unused areas (look there: isn't that Santa Claus?).
capability of sending lots of data on the Internet. Anywhere (our customers are anywhere!) and you can't see what that data actually is, it's encrypted (our customers value their privacy!).
Just the two things above are enough to:
- scan your whole hard disk and look for home banking credentials, credit card details, all sorts of valuable personal information and so on.
- turn your PC into a spamming relay.
Both these "features" would be quite valuable to someone, and they might be amenable to pay good money.
Especially since they needn't pay you anything. After one month service they can just disappear or say your services weren't up to spec. They might even ask for a fee to waive end-of-contract full hard disk sanitization (i.e. cancellation) as specified in a contract not everyone would read or understand. Or they might increase resource consumption so much on your machine, that you will ask for cancellation (and therefore be uneligible for any refund).
Another possibility is that the piece we're missing is "storage space at market prices". What if it's a different market? What if what they're after is completely untraceable disk storage, placed in dozens of separate private homes overseas, very difficult to even track, let alone seize or impound? Again, I can think of several people who would pay good money in order to gain this level of protection. What they would store on this fragmented data space is better left unthought.
And it still would be possible to lower costs by not paying the disk donors. Any software architecture that's capable of dealing with people turning off their PC's whenever they like (and they're saying this architecture exists, and they're using it) is ipso facto capable of dealing with people turning off their PC's because it dawns to them they're being ripped off. Therefore, I can't say that it is, but I believe that ripping off people could be a viable business model.