Some guidebooks are wrong, they now charge US$10 (80 bol) from Extranjeros (locals/local students pay 10/3 bol). Getting there is easy. Catch a bus/micro from the cemetery area of La Paz - expect a 90 minute drive, and expect to spend half the time picking up more passengers and escaping the traffic of El Alto. Price: 8 bs. You can also book a tour through the many tour operators in La Paz. Most of these appear to be in Spanish only.
The remains of a Pre-Incan civilization, the last skeleton recovered was 13,000 years old, this place is quite magical.
In August of 2007, a large hotel was being built in the town near the ruins. It will probably be completed in another 2 months or so but, there are about 2 motels there that seem fine. There really is no need to stay the night as visiting the ruins should take 4 to 5 hours the most. Perhaps in the future, there will be more to do and see, for now there isn't.
You don't want to leave this area. The area surrounding Tiwanaku and the small town around it is quite barren. You'll see a small mountain range to the West of the ruins and other mountains all around. The main attraction is this site so don't wander around.