You can camp for free at the 3 marked campsites near the river bed. The waterfalls are very loud!
When you get there people show you where you can camp and they als for 50 pesos. They own the cabañas near the camping sites. We just said ok and went to set up camp. Later he came again and asked for 50 MXN and we said yes, just give us a receipt. (Dame una nota por favor) he said ok in the morning then and never came back.