Shuttles
You can book online a private from the airport web page. The cost for a roundtrip in a private shuttle goes from +/- $135 usd (up to7 pax) to +/- $355 usd (up to 14 pax)
http://www.cancun-airport.net/services.php
We couldn't find the price on line for the shared shuttles (everything shows the same price as if it were private, but shared... so not worth it).
You can buy the shared shuttle tickets at the airport. We don't know if they are that much cheaper, but as can be seen from a website*, the round-trip price per person to hotel Dreams Tulum in pesos is $850, and in dollars $79 (so more expensive to pay in dollars than in pesos). The chances are that in the airport they also have a price in pesos and a price in dollars (which didn't go down when the pesos sank against the usd).
*
https://www.cancun-online.com/Traslados/Cancun/?asoc=adcu (pesos)