If you go and buy a product such as "olive leaf extract" from walgreens store for only like $ 4-16 does it mean its wont work or it can be dangerous?

Quality is the issue. There is no evidence the olive product is harmful. It is really a matter of quality and strength. I is much like a car, you can travel in a junker or a new mercedes. You may get where you want with either vehicle but you will more likly get there with speed, eas and comfort with the mercedes.