I don't remember the exact cost of flying but from Cozumel it was something like $150 and the ferry/bus was $100. Probably $50 more or something like that but worth every penny. Went back and looked at my pics and the airline was Aeroferinco but not sure they are still around. It was a fun flight, the twin prop (think Jazz flights to Edmonton) was Russian made and had all the Fasten Seat Belt signs in Cyrillic. I did find private Cessna flights but they were $300 a pop at least.
Tulum is the simplest ruin to get but I don't think it's got the same feel at Chichen. Chichen is one of the top ruins. I haven't been to Coba but it looks pretty good.
Note the lack of people in my shots.
For snorkelling you can find any number of places in the ocean. Akumal is likely the best area but there are spots all up the coast. Best is to do an internet search for spots near your hotel or hit up tripadvisor etc for reviews from people who were at your hotel.