The possible reason why she got offended by the money, Is that she thinks that you treat her like a hooker. She wants to be loved for who she is, not because you love having sex with her. Even if you didn't intend on saying it that way - Girls are like it. They take everything you say as a big deal.
You need to apologize to her in a meaningful way. She didn't accept the money because when you make love, you do it for the sake of making love not money. Now you probably might have blew the relationship, but don't give up yet.
Once she apologizes, think long and hard about what she said. Then it's up to you if you want to continue fighting for the girl of your dreams or not. Other then that, like Mama Di said, consider it as a life lesson. Get a new girl friend and move on in life but don't pay her again.
Mistakes are proof that you are trying.
A life spent making mistakes is not only more honourable, but more useful than a life spent doing nothing.