You said, "but way far from what was posted," yes it's ok to leave and tipping her twenty bucks for her trouble is absurd. If the provider is intentionally deceiving clients, don't give her a dime.
If there is a slight difference and it's still a deal breaker for you, then perhaps a twenty for her trouble is indeed in order. But if things are "way far off" like you asked, you are under no obligation to see her, nor do you owe her a dime. On the other hand, I've had providers scream and cuss at me for everything I was worth as i left, but oh well.