Hi!
Everyone is correct, the public bus is by far the cheapest. However, if you don't speak Spanish or if you have limited time - this won't be your best option. I took the public bus once (at those prices, had to try!). Luckily, my friend spoke Spanish, because none of the bus drivers or the guy that sold us the tickets did not. And it took ALL day. If you can get an express bus, that isn't bad, but we had missed the last express for the day. We also had to change buses at the half way point - which again would have been horribly confusing had my friend not been able to guide me.
Now when I go, I usually take a private shuttle. NOT the cheapest way, generally about $130 for the places we went - but once you divide it up between your party (if you are going with a bunch of people), its well worth it. They pick you up at the airport when your flight comes in, and they take you directly to your hotel. Its so much nicer. Its faster (fast being relative, after all this is Costa Rica and its very mountainous) its more comfortable and its really worth it if you are traveling with a group.
The shuttles service I use is called Costa Rican Shuttle (or something like that) and you can book right on-line.