Our Cast do this journey once or twice a week, I'm not sure if you know where Azul Sensatori is, but its a left turn by a petral garage with an Oxxo, its maybe 5km from the main Puerto Morelos round about. They pay 240 Peseo on average, The cheapist they have had is 200 peseo. Like I say add a bit more its about 5km short of the main turn off plus where you want to go from there... No more then 300 peso I wouldn't of thought. Just to add the best price we have had from Playa Del Carmen to Cancun is 340 Peseo.
Dougout, There is a place called Molino on Tulum, about 2 blocks past the bus station on the same side of the street. The taxis congregate in some parking spaces between the main road and the service road. This is the best deal going for getting to Playa del Carmen, about $240 per cab. They take off when it is full or when the full fare is met. They go downtown to the bus station in Playa. Not sure when you were here last but there is lots of road construction on the highway. There are overpasses at the Pto Morelos turnoff and over PDC as well. Thru traffic goes much faster but they are still working out the kinks of the service roads. My bet is you could grab a cab to downtown Pto Morelos for about $200 and maybe $150 or so to the bus stop on the highway, where you can catch a cab for a few bucks but I don't know how much. Next time I am in that area I will stop and ask the prices and get back to you, but please let me know if you would NOT want to come downtown. My bet is that a pickup at La Isla, would be a bunch as you will be perceived as a rich tourist. If you come downtown you will be thought of as thrifty, like staying in a lower priced hotel or even a hostel. Molino is a well kept secret. Not too long ago they painted out MOLINO from the top of the building but it is still a no brainer to find and these guys are already used to their routes south.
It's been years since I took a taxi to Playa, I want to say 3 of us paid $40 US one way. If a good fare is about $25 for one person, what are they charging for passengers #2 and #3?