I don't know necessarily about an STS kit being cheaper than ZZP's setups. I'm a fan of keeping the turbo in the engine compartment, not in the back of the car. I know that they can make power, but I just don't feel comfortable with them. I don't like having the life-blood of my engine taking a 30 ft journey under my car.

Also, STS holds a bunch of patents on the concept so I don't know that other systems can be built and sold without infringing on their patents.