I spent a week boating with diego (mentioned in el flaco's post) in december,and would highly recommend him.
We paid him around $60 dollars a day for a shuttle (split between 6 of us) and ocassionally had him run the the river with us as a guide, which he would do for just a few dollars more.
He's a great guy, a solid boater, and i really liked working with him. He charges far less than the gringo outfits and i kind of like the idea of giving my business to the locals. His english seems to be very much improved, since el flaco saw him last. If you are solo, check with him, he may have some others in a similiar situation to make it more economical for all. He doesn't require a weeklong committment to work with you.
He lives in turrialba.....his contact info is.....
Costa Rica Kayaks excursions
email is firstname.lastname@example.org
phone #(s) are ...... (506) 556-4679 / 363-4539