We too just visited Costa Rica! Got back this morning!
You can use dollars (USD) or colones. The ATM just after you go through security has the lowest fees to pick up some colones. As of this past week most people were exchanging 500 colones for 1 USD.
Make sure you use small bills, most places take credit cards.
When you exit it's $29 (USD) per person. When you walk in the airport it's to your right. You can't even check in without paying the fee. There are plenty of people working, the line moves quickly and it's easy to do. Don't pay with credit - it is processed as a cash advance not a credit charge. This is something no one talks about - it's actually written on the boards when you walk up along with MasterCard advertisements.
So if you are not reading carefully you see MasterCard and assume - great they take credit and they do - they just charge more for it.
Visa charges the lowest fees in CR
American Express is outrageous so we left it out of the equation and most placed don't accept it I think we heard 18%.
MasterCard was higher than Visa so we used Visa at most restaurants and excursions. Our hotel required cash deposits to book events - not an issue - usually $20 USD per person.
Edited: 9:59 pm, April 29, 2013