During Chichén Itzá’s opening hours 1st- and 2nd-class buses passing through Pisté serve the ruins (hail the bus and check with the driver), and they will take passengers from Pisté for about M$6 when there’s room. For a bit more, 2nd-class buses will also take you to the Cenote Ik Kil and the Grutas de Balankanché (be sure to specify your destination when buying your ticket). If you plan to see the ruins and then head directly to another city by 1st-class bus, buy your bus ticket in the visitors center before hitting the ruins, for a better chance of getting a seat.
There is a taxi stand near the west end of town; the price to the ruins is M$25. There are usually cabs at Chichén’s parking lot.