I was in the exact same senario when stationed in Ohau, Hawaii there was a 8 hour weekend Family cruise to Maui. The GF chose to fly in and out and since a free ride was provided and she chose other means she had to pay.
But then again I dont know what your income situation is but if you make much more than her pick up the tab, but if she can afford it exercise some equal rights.
On a side note, I conducted an experiment with the same GF mentioned above.
I was a junior enlisted making chicken scratch and she was a nurse making $28 an hour. I was ALWAYS picking up the tab, groceries, dining out, entertainment, etc.
So one day we were walking home and she says" I feel like Mexican tonight"
I say "well its going to have to be on you, cause Im broke"
She replied quite snippy"You dont have $20!?"
"Nope" said I.
30 seconds pass and she says "ok, we have plenty of food at home we can eat."
Cant say I was surprised, needless to say we arent together anymore--selfcentered biotch.