SV,

Yes it's all a calculation. You have to put a roof over their head and she has to have a vehicle to drive, plus food, clothes etc.

The kicker is that doesn't even include sports, braces, medical bills which you will most likely pay the majority of in the future.

Your best bet is to sit down with her and try to explain to her that you can't afford to give her $3k a month. That is your best bet.