With the Flat Deposit extension you can enhance the built-in deposit options and charge a flat deposit rate at the time of booking. WP Hotelier already offers the possibility to require a deposit before to confirm a reservation. But the deposit option included in the core is calculated as percentage of total price of the reservation.
To setup a flat deposit on a specific room, edit it and select Flat rate in the Deposit amount dropdown (the Require deposit? option must be checked). Then write your desired price in the Flat rate price option.