From the very nice Cancun bus station in downtown Cancun, you can take a bus to Chichen Itza. I believe the actual bus stop is called Piste, but you can get off the bus at the gate to the ruins. Look it up on ticketbus.mx. There are "luxury buses" which stop less and get there faster, big plush seats, cold AC, very nice. You could make it an overnight in a better town Vallodolid and be able to also see Exmal, another closeby ruin that has more recently begun excavations, plus a major cenote in walking distance of town. Your transportation to the ruins from Valladolid would be cheap if you use, I forget the term, but a local minibus. Hotel Meson Marques there is cheaper than Cancun and nice enough that Jimmy Carter stayed there several times.