Prius mileage increase modification

I am doing a survey for a capstone engineering project. Our product is an attachment to brake pads on the Prius.

The product is able to increase the Prius by 6 mpg, and will cost around $600 to install. If the product takes about 5 years to pay it off, will you be willing to purchase the attachment?

I am also looking for comments, both pos and neg, about such a product.