It depends on a number of factors. In the past guests paid the full amount up front. About a year ago AirBnB rolled out a ‘Pay less upfront’ option for guests. Whether or not they can do this depends on a variety of factors including overall price, what current they are paying in, how close the booking is to the arrival date. If they can use this option it will be displayed as a tick box on final payment page - if they tick the box they only pay 50% upfront