Unless you told her before she moved in that she'd only need to pay $200, then she should be paying half.
If you told her that $200 was the deal, then it's not her fault your circumstances/lifestyle have changed.
I'd personally pay half because I like to have an equal balance of power - get my name on the lease, etc - but if she left another situation and part of the allure was the cheap rent, well, you made a mistake my darling. Either try to talk to her and get her to pay more, suck it up and learn your lesson, or sweetly ask her to go visit mat and babushka for an extended period of time.