Except for the shuttles operated from the youth hostels, there are no colectivos out to the beach. You either hitch, ride a taxi, rent a bike or walk. And it’s a long walk.
Bicycles can be a good way to get around and usually cost M$150 a day.
Taxi fares are fixed and pretty cheap; from either of the two taxi stands (one south of the bus terminal, which has fares posted; the other four blocks north on the opposite side of the street) in Tulum Pueblo to the ruins is M$35. Fares to most cabañas mentioned here are M$35 to M$50.