You don't state your nationality, but if you're Austrian, unless you have a visa issued by the Costa Rica government you must have a ticket to leave within 90 days. It's the same for U.S. Citizens.
The information is clearly stated here:
http://www.costarica.com/trip-planni...a-information/
Towards the bottom it defaults to "Tourist", then you select your nationality from the drop-down list.