Registration fees cover the forum costs, cultural visits, and food (sandwiches for lunch and diner in restaurants).
Accomodation and transportation fees are not covered.
The best time to book air tickets is usually 52 days in advance (late april).
Thanks to the Paris YWAM base, some inexpensive rooms to share are available for the first ones to ask.
Alternatively, sharing an AirBnb might be the best solution.