You have access to all the information and the accommodations of the website completely for free.
You can also communicate with the landlords in order to ask questions through the website (it’s not possible to exchange contact details before the booking has been done), without making any payment.
The only payment to be made at Erasmusu is at the time of booking. Erasmusu will ask you to pay the first month of rent as well as a small service fee that will be different depending on the country where you are booking.
This service fee will only be paid once at the moment of the booking, it won’t be required every month.