Re: Payment, at $0.015/GB
The $0.015/GB is per month, but this is the price the person storing the data pays, and compares to the price to rent storage on S3 or Glacier, etc.
As mentioned by Martin Summers above, the amount you get paid for sharing your disk space is based on some obscure scaled formula calculated at the end of each month. But obviously less than $0.015/GB/month, probably a lot less if they need 2x or 3x redundancy.
Apart from the other concerns, you may need to balance any revenue with extra electricity costs if this makes your PC run in non-standby mode for longer.
I also suspect that if this became a large business, some of the ISPs would start noticing customers making money off their bandwidth and might try to add T&C's restricting making money off consumer ISP connections or suggesting that some of this revenue should go to the ISP.
Finally, although they compare their $0.015 to Amazon S3 at $0.023/GB, Amazon also has "S3 infrequent access" at $0.0125, i.e. cheaper than Storj and possibly more reliable because you're not relying on random people's PC's. So it's not clear they are going to win here...