Can Breaker Bay Grid be used to reduce cost of other Regions?
You cannot because upgrades are always installed un-rezzed from your hand.

In order to put SanSan City Grid onto a server you first have to discard Breaker Bay Grid. Therefore by the time you can rez SanSan City Grid you no longer have the effect of Breaker Bay Grid.

Also note the reason is because you aren't allowed to have two regions installed and the time to discard the other one is before the new one is played per the rules.