I like the idea of a builder having discount to the factory that it belongs to itself. Like mentioned, unit sharing is already possible so it should not really affect team balance. In my opinion it would be much better if not every player needed to build their own striderhub and what not when they want to get access to more units despite teammates already having access to those units.
I think it would be nice if there was somekind of togglable option on factorys to share them for the rest of the team without sharing everything else. Only the player who "owns" the factory is allowed to decide whether it is shared or not and units queued by other teammates could have their own team color surround the ordered units icon in the factory queue.
Of course here is some problems:
* Who pays the metal cost of the unit? Preferably the person who ordered the unit should pay its metal cost, but I don't see this being easy to implement.
* What to do if someone wills your factory with unit orders of their own? It would be good if the owner of the factory had ability to remove build queue requests from other players while others are not able to remove the factory owners queue requests.
* What to do if factory is set on repeat when someone else queues unit? Preferably, all queue requests by players not owning the factory should not be repeated.
Still, that is probably too much work and the discount factory idea will help to fix the same issues.