@Francisc0, @David126, @Ira4 A host may change his pricing anytime up to the time a guest makes a reservation request. At that time the price on the listing is what the guest is required to pay. Asking for more money other than what was on the listing at the time of the request is a violation of Airbnb's Terms of Service. Here is the relevant section of the Terms of Service:
"Other Members will be able to book your Accommodation via the Site, Application and Services based upon the information provided in your Listing, your Guest requirements, and Guests’ search parameters and preferences. You understand and agree that once a Guest requests a Booking of your Accommodation, you may not request the Guest to pay a higher price than in the Booking request."
Of course the host can decline the reservation request, but it will count against their committment rating. Also, if a guest suspects that the host is doing a "bait and switch" tactic, the listing could get flagged by the guest.
Doing this is not good business practice.