She can't pay. Since her country treats their highlanders/pilots like country property/slaves, the moment she went MIA she was considered DEAD. All her assets and wealth has been confiscated and her official status as a citizen is now treated as dead. That's why the whole drama between Yui and Yuuta being enemies is weird in the first place. Yui doesn't seem to realize she is working as a soldier for an invading country that treats people as weapons and disposes those not useful (like they thought with Yuuta), as slaves even though stuff happened right in front of her face.