A small correction here. The plates of a capacitor are merely conductors. The charge is actually stored in the dielectric as an electrostatic field.A battery uses a chemical reaction (reversable in the case of a rechargable) while a cap simply passivly stores charge on the surface area of its plates.
Al, I was working from the charge cost figuring electricity at 0.06/KWH. Nine bucks equates to 150KWH used, which for a period of five minutes is 12.5KW used. That translates into 52 amps at 240v. So, the charge isn't bad electrically. I'm pretty dubious about the stored charge to horsepower output as others are.
Dean