If you happen to be in Tortuguero around May, don't waste your money on turtle spotting.
There are three kinds of turtles that lay their eggs on the beach of Tortuguero: leatherback turtle, green sea turtle and hawksbill turtle. From February to July it's the leatherback turtle that comes to visit the black beaches of Tortuguero.
At night you can go for a guided walk to spot the turtles. You walk around the beach in total darkness ( no light-coloured clothes, no lights, no photos) for about 2,5 hours looking for the turtles. However, as you only have 60% chance to see the leatherback turtles, you will likely come home spending 10 USD without seeing anything. Two days in a row, several groups in our hotel didn't see them.
It is said that the best time for turtle-spotting is from July to September when the green sea turtle comes ashore.
I went to Costa Rica 14 years ago and in Summer 2004. Both times I went to see the turtles lay their eggs in the sand. It's completely different now. Everyone sould watch it, but 14 years ago we were 6 or 7 people in the whole beach and last summer we were a hundred waiting in a plane track to enter the beach in turns. Now you are allowed to see the turtle for 30 seconds at most. Again, everyone should watch it but remember: it's short and sad.