In Pretty Woman, Vivian tells Edward that he couldn't afford for her to stay with him for the entire night:
Vivian: So, I'm on an hourly rate. Could we just move it along?
Edward: Somehow I'm sensing that this time problem is a major issue with you. Why don't we just get through that right now.
Vivian: Great, let's get started.
Edward: How much for the entire night?
Vivian: Stay here? You couldn't afford it.
Edward: Try me.
Edward: Done. Thank you. Now we can relax.
Edward picks up Vivian in a Lotus Esprit and takes her up to the penthouse at the Beverly Wilshire hotel. Given both of these things, it is easy to ascertain that Edward is rich. Vivian is aware of the value of the car due to her knowledge of automobiles. She is also visually nervous walking into the hotel because it's an establishment for the upper class.
Vivian already knows that Edward is wealthy. So, why would she think that Edward couldn't afford $300 for her to spend the night?