Quick run down if you ever go: Don't order tickets online. You can pay around 30/person and it's cheaper if you buy tickets once you get there. We took a tour bus that was 15/person with a wonderful tour guide who pointed out sights on the way. She was super nice and funny which made the 40 minute drive go by faster.
The cheapest way to go? A taxi. They charge $40 to take a group of up to 4 out there, they will let you stay as long as you want, and then bring you back. I'm not sure how nice of a trip out there it would be.... as far as the driver being a tour guide, but if you want the cheapest, that's it.
This was our 2nd time here and the first time, we stayed more in the city. This time around, I learned, there is be a u t y e v e r y w h e re you go. Sometimes, you just have to travel a little farther to find it. :)