Is Bangkok Airways the same price to book a flight at the airport?

NO prices keep changing

They normally have a few different fares to most of their destinations and the cheapest fares often sell out. You can book tickets online up to 24 hours before the flight. I'm not sure if the price is the same at the airport or not, you can call them at this number to find out:
Ticket /Reservation : 66 (2) 134 3888-89 (Ticket), if you're in Thailand the number to call would be 02 134 3888-89.

on-line price used to be a little bit lower than to buy at the airlines counter..

PEACE =D

You can check at http://www.relaxthai.net/air-ticket-bangkok.html