On our last return trip from Costa Rica we were introduced to what I now understand to be a common scam. When disembarking from the bus at the Caribbean terminal were encountered the expected mob of taxi drivers. We asked "how much to the airport" and were pretty much unanimously told $20 US. This is pretty much in line with what we had paid in the past. Once we were loaded up and on our way the driver again asked where we were heading. When we told him the airport he asked which one. When we told him that we were heading for the international airport he acted surprised and told us that it would be $30 US. We told the driver that he should have made it clear at the start and we gave him20$ and what we would have normally given him as a tip and listened to him argue as we walked away.
Since we were a large group we were in two taxis. The same thing happened in both cabs. Make sure that you are clear in which airport you are heading for ( I don't think that a lot of people are heading to the airport at Parvas but I guess it is possible)