hostelworld.com

There are tons of hostels in Costa Rica, and pretty much everywhere else around the world. A private room in a hostel in CR will run you around $20 per night. So, waaaaaay cheaper than a hotel. I stay in hostels all the time, I don't even consider hotels anymore. Much more character, friendlier people, and usually a lot more ammenities. (At the last hostel I stayed in, the owner cooked me a huge (and I mean HUGE) breakfast every morning. She also woke up at 5am on my last day to make sure that the airport shuttle arrived on time and that I made it on it ok. What hotel will do that?)

As for rental cars, you can get one, but the bus is so cheap, I would just take that. It's more fun anyway. Liberia is closer to the Pacific beach resorts, San Jose to almost everything else. Get the Lonely Planet Costa Rica, it'll tell you everything you need to know.