Wow. Incredible. We booked this trip through the resort. A little on the expensive side ($75). It was a half-day trip that included the bus ride (3 hours), English-speaking tour guide, and breakfast.
This was really fun. Because none of us were familiar with Cancun, we chose to book through the hotel.
I think this trip can be done on your own, however, with the right planning you could probably save a lot more money.
Our guide was excellent, and very knowledgeable about Mayan culture (being Mayan himself).
Incredible ruins, and local craftsman/women have some really incredible things to offer. Personally, I bought a jewelry box, and two dishes, as well as a carved mask. I probably spent about $30.
Make sure while you are there you check out the natural sinkholes. The area is mostly limestone, so over time nature has carved out some gorgeous and breathtaking natural swimming holes. Some of them have no way out, however, and Mayans used them for sacrifices!