Hi there,
Down in Playa del Carmen there is a bunch of little inexpensive beachfront places and one of them is called 'diamante' where you do not make a reservation to stay and offers decent huts on the beach.
Another alternative is Tulum Playa which is less touristy than Playa del Carmen and you can tent or
stay in beach huts for as little as $5 a night.
Hope this helps
VacationBuddy