Exit ticket needed for visa?

Karen,
The answer is yes, but, you can go to the Copa airlines website and simply print a "reservation", and that will work.  You don't have to pay anything.  Just try to print a screen with your names on it.   
You can also buy a bus ticket to Panama or Nicaragua via TransNica or TicaBus for about $27 per person. 
The departure check in in the US is where you'll most likely be asked for your "return ticket", or proof of traveling beyond Costa Rica.  In this case, a Copa Airlines reservation has always worked for me.   
Immigration/Border agents are very inconsistent in Costa Rica.  Even at the San Jose airport.  You may run into an agent that wants all the details about your travels here, and when you're leaving, or you may get someone who looks at you for half a second, stamps your passport and wishes you "Buen dia". 
Whatever you do, get your "reservations" out of the country for 85 - 89 days after your planned date to enter the country, because some agents actually calculate the number of days until your reservation to leave, and write that on the Visa stamp as the number of days you're allowed to be in CR.  Other agents won't write anything, so the "90 days" is understood. 
Our first time coming into Costa Rica, we printed Copa Airline reservation for about 60 days from date of entry, and thought we'd be given a 90 day Visa.  But no, the agent wrote in 60 days and that overrides the blanket 90 given to most tourists. 
The immigration agent's decision at that moment of entry supersedes the law.  We've actually seen that in writing!
I hope this helps you.