I'm from Bulgaria, where Tribulus was originated.
Here Tribulus (more popular as Tribestan, the product of our major pharmaceutical company Sopharma) is widely used by athletes. I even know some people selling the herb (not the extract, the whole plant) for prices as low as 11-12$ for a kg.
Many people tell legends about how it increases your strenght and it's anabolic effects. These legends are more than exaggerated.
The true purpose of Tribulus is more to recover your natural testosterone production after a cycle with androgenic steroids. It's also know as the "bulgarian viagra". It's true that it helps sexual potency. Tribulus is natural and side effect (when in normal dosing) are yet to be discovered.
Here it's definately worth the money, because it's extremely cheap. I can't say anything about tribulus versions in the USA...