Unless you speak Spanish well or she speaks English, just be to the point. No need to go beyond the typuical small talk that they all wil try to engage you in. They'll start with the typical questions: your name, hotel, if you've been to CR before (answer is always YES btw), how long you in town. Just play along, but eventually she'll ask if you want company. If interested, reply with what you are looking for, and what you are willing to pay. Then shut up and let her decide if she wants to acccept your offer. Don't ask her how much because it will always be $100 (or more). You are not on a date, no need to try to romance them. You don't need game. Save that shit for the gringas back home.
If you are not interested in a chica don't talk to her. Be polite but firm, "No gracias" works well. No need to waste her time or yours on pointless chit chat.