The problem with that is that K costs in FS are already really high. Are you going to price it like the Dreadnaught unlock? I can't see very many people ever using that, because the Dreadnaught itself is so useful and if you are at that point, you can probably just build the transceiver and win without taking more systems to build the super expensive fortress unlock.
I was actually suggesting pricing it HIGHER (20k) than the Dreadnaught unlock. As I already wrote, I specifically wanted to make it unattractive/unavailable to people just trying to win a Fallen Spire game with a few cities, while providing defensive firepower for those people who go on producing more spire cities and conquering loads more, to give some progression and something to look forwards to.
Think of it as an optional super defense if you are mad enough to play Fallen Spire like a conquest game with really high AIP building up to a massive strike on the AI homeworlds, which is one of the options strongly recommended when the fifth city is constructed, rather than sticking to the minimum needed to win via Transceiver, and as something almost out of reach tempting players that want to win with the Transceiver.
Quite possibly not many people would ever use that. But it would provide a nice sense of progression for those that did. Whether that's worth the time and effort to implement is a completely different matter, but that, as always, is up to the developers. As players, we can but dream.
EDIT: Clarified usage.