First, imo, you are being extremely thoughtful and helpful. Above and beyond. Asking for half or less of the fare she could get on her own is reasonable. You are giving up something of higher value. Of course you could have been even more generous had you chosen, but you had no obligation of any type to offer to help in any way at all.
Sounds (reads) like her reaction to you and your offer was knee-jerk. And maybe even recent, like today?
It would be interesting to see if she reconsiders after having a chance to relate the situation to a friend or three. She may get some advice that suggests she overreacted and did not think of the bigger picture here, was being rather self-centered, while you were being generous without needing to be or expecting anything outside of the money in return, etc.
She may then come to the idea on her own(!) that she should approach you again, a bit more humbly, and ask if the offer as extended is still available. Then it is on you to decide next steps, and again imo, there is no wrong answer on your part. Yes or no, you have either a generous heart or reasonable justification.
As others said, based on her displaying such an entitled and ungrateful 'tude in her unfiltered reaction, she may be a tough companion for such a potentially wonderful trip. Her asking you or expecting you to pay for meals, transportation, etc. is a reasonable concern that she brought on herself.
At this point, whatever you decide to do or not do next cannot be criticized, imo.
