Given the Internet, the cost of labor for distribution of digital goods over that is cheap as all hell when compared to the cost of labor to distribute any sort of physical good because first you have make copies of this physical good, which starts off by requiring additional raw material input.
Say we've got this mp4 file that everyone wants. Let's use bittorrent for our digital distribution. So we need to: create a torrent, upload it, and then keep their computer on for a couple more hours? Let's pay $400 for the job. I think it's well within the realm for a clever teenager to do it, or a rando you find off of Upwork or Fiverr, that who knows what they're doing, to finish the first two steps in an hour, so I think that rate is plenty generous. But let's also pay for their Internet connection for the month ($100), and a cheapo laptop to do this work on ($500). This brings our digital distribution costs including the cost of labor, using bittorrent, to be able to make billions of copies to be... $1000?
Even if you pay an engineer to do that job, labor doesn't get residuals, so aren't paid for each copy made, so the cost of labor, whatever it is, is essentially flat. Compared to if you were trying to copy and distribute a physical good, the more copies you make, the higher your costs and $1000 just isn't going to get you far at all.
I'm more interested in what you mean by interest rate though, mind explaining how that fits in here a bit further?