I'm irritated at my friend for paying full price for someone else when she didn't for me.
Why do you think she didn't for you?
I think she's prejudiced against poor people.
That's a high accusation. You shouldn't care how other people spend their money anyway. It isn't your business.
If she fucks with me she will lose a good friend.
If it bothers you, it's better to politely ask her about it.  There might be more to it that you didn't know.