I did the San Juan horseback riding in the rain forest last August and it was a lot of fun. I think we paid around $80 per person, but I don't remember exactly. We booked through the cruise line, mainly becuase I'm overly cautious with that type of thing and wouldn't have felt safe otherwise. Yet, I know that many other people book with non-affiliated tours, so that is your call.
My big complaint about the excursion was the time factor . . . The discription said it was around 4 1/2 hours, which was true, but they failed to mention that you have to take a 1 1/2 to 2 hour bus ride EACH WAY to get to the ranch! We didn't find this out until we were on the bus and it came as a big disappointment. The actual time spent on the horseback tour was only about an hour or so, when I was expecting it to be a few hours long. Other than that, it was great, just be prepared for the long bus ride!
Also, be sure to wear jeans and a pair of sturdy boots, even in the heat, you'll be happy that you did. Your legs will most likely get brushed up against trees, bushes, other horses/riders, your feet will be sore from the stirrups if you wear sneakers and you will get a little dirty.