Tips: You don't need to know swimming to get in there, some of the water is as much as 15 feet deep but you can float using the life vests given.
The entrance fee was $67.15 and it has to be booked at least 5 days in advance to guarantee a slot. They also charged an additional $60 for the photos, which I felt was expensive, but there's no camera (even if it's waterproof) allowed inside the caves.
Also, try using 'Taxi Collectivos', these are shared hop-on/hop-off taxis located between 15th and 20th St at PDC. They are the cheapest and quickest mode of transportation south of Playa.
After the tour I went and relaxed at Akumal beach, which is about 30 min south by 'taxi collectivo' from Rio Secreto.