On the efct17 there has been a discussion with Fairphone employees and community members about this topic and that is where the number of 50.000 comes from. It was pretty clear at the time that there was no viable way to get these batteries.
We did ask them on the EFCT. Batteries are not that cheap. They did a calculation like yours and the total price was nearing a million (or over a million, I don’t remember) instead of your €250.000. Batteries are simply not €5 even if you order 50.000 of them. And that doesn’t even consider the environmental problems associated with ordering 30.000 or more useless batteries.