It's actually a bit more complicated than that. You get 5% of your data back for free every month but it's prorated at a data transfer rate of (5% of your data)/num days in month = data per day. if you store 1TB you can retrieve 50GB/month = ~1.66GB/day.

If you go over that it's more expensive but they prorate it based on how much free data you get and how fast you were going while you downloaded it.