There are 2 or 3 international car rental companies in Uvita (budget, alamo, and I believe hertz). However, if you want long-term cheap rates, than look for Tricolor Car Rental (http://www.tricolorcarrental.com/) and Adobe Rent a Car (http://www.adobecar.com/) out of San Jose. Adobe Car even offers additional discounts if you become one of their friends on their facebook website. A friend has rented from Adobe several times, and she has been very happy with their value and service. For long term rates, the most basic 4WD starts at $20 per day!
I first came to Costa Rica in 1998. I fell in love with the country right away for so many reasons. I have been coming, living and unfortunately never staying off and on since.