TGG, listen. You do this and you'll never gain any respect from yourself, or respect from other people. I give you 95% odds that everyone else thinks this girl is a b*tch too but are too afraid to say anything. Nobody is going to turn against you...if anything, they'll respect you for doing what they couldn't.
You don't have to give her some huge explanation. Just look her in the eyes and say, "B*tch, I told you from near the start I can't go so the bottom line is, if you want that $75 go find someone else in a hurry." Then wherever you are, get up and leave. You'll feel a lot better.