I was at Torrey Pines and happened to be their just as the guided tour was starting. I tagged along, the trek was about 2 miles. The guide would stop and explain the importance of each plant and give a brief history of the region.

They are 100% free.

Guided tours are Saturday and Sunday. They leave from the Ranger Station (Which is at the top of the hill at the entrance of the park. The tours leave at 10:00 AM and 2:00 PM and last for about an hour to an hour and a half. The guides are volunteers who really care about the park. It is a great way to pass some time in a beautiful spot.