Actually the majority of species from Costa Rica that you see in the US trade, like red-eyes for example, are also found in other places like Panama and Nicaragua. Its from these countries they are imported. Costa Rica is very protective of it's native species. So to answer to your original question, no, its illegal. Sorry, if I came off all "hellfire and brimstone" the first time.

Even if Costa Rica let you take things, US Customs would not allow you to bring it back into the country. It woud be the same no matter where you were coming from outside the US.

Now if you want actual Costa Rican frogs, and I suspect you will when you get back, you may be able to get them frog Understory Enterprises. Brian Kubicki of the Costa Rican Amphibian Research Center in Guayacan, Costa Rica is captive breeding some of the native species from the area and shipping them to UE to raise money for his conservation work in Costa Rica. All is on the up and up. The downside is UE is in Canada. Yeah for the Canadians! Fear not, I heard that there are shipments to the US, but I don't know how that works.

I am hoping to get my hands on Centrolene ilex someday which I hear is a very common species down there, but unheard of in the US trade.