I don't feel like splitting the cost of an expensive hotel with my partner.
Why don't you want to split the cost of the hotel?
Getting the hotel was her idea, so I think she should pay for it.
It's rude to not share the cost of the hotel with your partner if you're both going to be staying there. I think you should pay for your half of the room, because that's the right thing to do when you're in a relationship.
I know what I'm doing.
Splitting the cost would be the fair thing to do, but if you think the hotel is too expensive then you should discuss this with your partner. The two of you should be able to find a compromise that works for both of you, that way neither of you feels taken advantage of.