Conquest shop Darth Bane

From what I've noticed, once a character drops to 400 in the conquest store you have a limited time to find it there.  As soon as the newest conquest series starts (newest character) then the cheapest one drops out and is solely available in proving grounds or other promotions.  At the moment, Queen Amidala is 400 conquest currency so you'll continue to see her pop up in the store until the next conquest character arrives after Jocasta Nu (so about 2.5 months from now).  Bane got  relegated to 'proving grounds only' as soon as this current conquest started.