When I travelled out to Tiahuanaco via micro (intending to put some money into the local economy) I discovered that the entry fee at the site was $80 bolivianos. However tour agencies in La Paz will charge a total of $50 Bolivianos, including transport to and from the site and a tour guide.
Unique Suggestions: Its your choice on how you spend the money, but check with a tour agency first.
Not really a tourist trap, but I found all the signs around where in spanish. No problem for me, but if you don't really manage well in this language, you better bring a dictionary. I think that for a relatively expensive entrance fee (10 USD) they should considering adding at least english translations to the signs...
Unique Suggestions: A good option is to take a local guide at the entrance. He will explain to you in english all the curiosities of this site (and there are many!).