Originally Posted by Jesus
As far as I know, it's free on Sundays, but just for Mexicans, in my past trip to Edzna (to come soon) they asked me to pay but I showed my Mexican ID and got through.
The plan is doable if you have the stamina,
You will get a feel of the road from Tulum to Valladolid and can decide on the return trip if you want to stop at Coba or spend more time at Chichen Itza.
Your planned driving times are for optimal conditions, but as said, take in account the speedbumps (Tope) and add 20% at least.
El Tajin was free for us when we went on a Sunday in October.