Build a booking widget with live pricing

Build a small reservation widget for a listing, front end and a simple back end. Pick check-in and check-out dates and the number of adults and children; show the total price, which depends on the dates and the guest count; disable the button when the listing is full; and show a confirmation dialog when the user books.

Follow-ups: how would you show prices in several currencies across the front end and back end; how would you cut down the number of requests the page makes; and how would you cache a reservation securely so it is still visible offline?