We stayed 2 nights in Cabo. Whilst camping in tent is more expensive than in Arrecifes (25k COP pp), the site is a beach where you can actually swim. The woman at the counter (who will NOT let you check in before 2pm) is not the friendliest, but once you have passed her, beauty awaits you. The restaurant is also great value for money (from 10k to 20k COP) given the location, and the beach is awesome. Not many people as facilities are limited is also an added bonus. Showers not very intimate, you may want to keep your swimwear on!
Cabo is about 1.5 hrs walk from the entrance of the park. Taking a bus from Santa Marta to the park is easy and costs 6k COP. Entry to the park is a hefty 36k COP pp. Then a minibus takes you further in for 2k COP, and you can then choose to walk (as we did, 45 mins to Arrecifes, 1.5 hrs to Cabo), or to take a horse.