Have you got an email confirmation with that price?
Technically the contract isn"t finished until you hand over the cash or you use the service (I think), so if they haven"t taken the payment then they might be able to change the conditions. But if you have the reservation confirmed in writing at that price I would be surprised if they insisted on a change.
EDIT: I don"t necessarily think it"s a mistake anyway, price goes up as they run out of rooms - you might just have got lucky and booked before a mass of other people did the same.