All I can say is you get waht you pay for with that.
If money was a little tight, but quality and potency was a priority than I would probably buy either Biotest Alpha Male or Nutrex Vitrix. Tribestan appears to be the most expensive based on dosages used, but I do believe it was the original product the Bulgarians used to raise natural test levels so I'm pretty sure it works very well.
If you're gonna go cheap on the tribulus then I would buy Optimum nutrition's tribulus 625 brand. It's like 15 bucks and it does work, but you gotta take about 6 pills a day compared to it's recommend 1-3 and there's 90 caps per bottle. So that's 300 bucks for a month's supply comapred to say 40-50 with alpha male and Vitrix which are both more potent and have additional ingredients in them to cut estrogen production down.
Quality isn't so much the issue with the cheaper trib products, it's the potency of it's ingredients.