Probably around $100/Farad, I don't think you'd need much more that that. There is good info
here on what capacitors do and how to pick one, of course this company says theirs is the best, but it might help you. Installing a capacitor would be easier, and cleaner, but you already have the battery and I'm not sure how you would hook it up, but you'd need to at least connect it to your equipment and the/an alternator to keep it charged. Plus I doubt you have room under your hood, and that's a lot of extra weight (if a fast car is important to you).