Flight one way to enter in Costa Rica

Yes, the same rule applies in Costa Rica, although some tourists buy an ongoing ticket then cancel it after arrival.

What we did was buy a one way ticket into Costa Rica online (such as expedia).
Then the night before the flight leaves, buy a one way ticket out of Costa Rica (make sure it's cancelable within 24 hours!).
Make sure you arrive in CR within the 24 hour cancellation expiration, and as soon as you arrive in the country and go through customs, cancel the one way flight out of Costa Rica.

You can buy an onward ticket reservation at the one way ticket  website for $19. It is valid for 14 days.

No, you must have proof of onward travel within 90 days.  I purchased a 24 hour refundable airline ticket the morning of travel and cancelled it after arriving in CR